AP e Unit 3 - Intermolecular Forces & Properties

AP Chemistry

Intermolecular Forces QuestionsISPS Chemistry Aug 2021 page 1

Unit 3 - Intermolecular Forces& Properties

3.1 Intermolecular Forces 3.2 Properties of Solids 3.3 Solids, Liquids & Gases 3.4 Ideal Gas Law 3.5 Kinetic Molecular Theory 3.6 Deviation from Ideal Gas Law 3.7 Solutions & Mixtures 3.8 Representations of Solutions 3.9 Separation of Solutions & Mixtures Chromatography 3.10 Solubility 3.11 Spectroscopy & Electromagnetic Spectrum 3.12 Photoelectric Effect 3.13 Beer-Lambert Law

AP Chemistry


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AP Chemistry


AP Chemistry


3.1 MC Questions1. The diagram opposite is a molecular model of a gaseous diatomic element that is just above its boiling point.

Intermolecular forces between the gas molecules will cause them to condense into the liquid phase if the temperature is lowered.

Which of the following best describes how the model is limited in its depiction of the phenomenon?

A It does not show how hydrogen bonds are constantly forming, breaking, and reforming, which results in a net force of attraction between the molecules.

B It does not show how the interactions between ions and the induced molecular dipoles result in a net force of attraction between the molecules.

C It does not show how the interacting permanent dipoles of the molecules result in a net force of attraction between the molecules.

D It does not show how the temporary fluctuating dipoles of the molecular electron clouds result in a net force of attraction between the molecules.

2.

The electron cloud of HF is smaller than that of F2 , however, HF has a much higher boiling point than has F2 . Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF ?

A F2 is soluble in water, whereas HF is insoluble in water.

B The F2 molecule has a greater mass than the HF molecule has.

C Liquid F2 has weak dispersion force attractions between its molecules, whereas l iquid HF has strong ionic interactions between H+ and F- ions.

D Liquid F2 has weak dispersion force attractions between its molecules, whereas liquid HF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules.

O

O

AP Chemistry


3.

Four different liquid compounds in flasks at are represented above. The table below identifies the compounds. Flask C shows the most particles in the vapor phase.

Which of the following is not shown in the model but best helps to explain why flask C must contain pentane?

A The random motion of the particles within the liquids.

B The relative speeds of the vapor particles in each flask.

C The strength of the intermolecular forces between the particles in the liquids.

D The structural formula of the molecules of the liquid and vapor in each flask.

4. In the diagram below, which of the labeled arrows identifies hydrogen bonding in water?

A A

B B

C C

D D

O

O

AP Chemistry


5. In which of the following liquids do the intermolecular forces include dipole-dipole forces?

A F2 (l) B CH4 (l) C CF4 (l) D CH2F2 (l)

6. The London (dispersion) forces are weakest for which of the following gases under the same conditions of temperature and pressure?

A H2 B O2 C Xe D F2 E N2

7. Thymine and adenine form a base pair in the DNA molecule. These two bases can form a connection between two strands of DNA via two hydrogen bonds.

Which of the following diagrams shows the correct representation of the hydrogen bonding (denoted by dashed lines) between thymine and adenine base pairs?

(In each diagram, thymine is shown at the left and adenine is shown at the right. The bases are attached to the backbone portion of the DNA strands.)

A B

C D

8. Which of the following is the strongest type of interaction that occurs between the atoms within the circled areas of the two molecules represented below?

A polar covalent bond

B non-polar covalent bond

C hydrogen bond

D london dispersion force

O

O

O

O

AP Chemistry


3.1 FRQ1.

The boiling point of liquid propene (226 K) is lower than the boiling point of liquid vinyl chloride (260 K). Account for this difference in terms of the types and strengths of intermolecular forces present in each liquid.

1 point is earned for a discussion of intermolecular forces and for a comparison of their relative strengths.

Both substances have dipole-dipole interactions and London dispersion forces (or propene is essentially nonpolar with only LDFs while vinyl chloride has both LDFs and dipole-dipole forces). Propene contains a CH3 group, but vinyl chloride contains a Cl atom. Vinyl chloride thus has a larger electron cloud, is more polarizable, and has a larger dipole moment. Thus intermolecular attractions are stronger in vinyl chloride, which results in it having a higher boiling point.

2.

a) At 1 atm and 298 K, pentane is a liquid whereas propane is a gas. Explain.

1 point is earned for recognizing that pentane and propane both have only LDF’s.

1 point is earned for recognizing that pentane has a larger electron cloud with greater IMF’s and higher BP (reference to state of matter).

Both molecules are nonpolar, and the only intermolecular forces in each are London dispersion forces. However, pentane is larger than propane and has a more extensive electron cloud that can be involved in a greater number of London interactions, leading to stronger intermolecular attractions overall. Thus it takes a higher temperature for pentane molecules to have enough kinetic energy (on average) to overcome their stronger intermolecular attractions, thus pentane has the higher boiling point.

AP Chemistry


Q2 continued

b) At 1 atm and 298 K, methanol is a liquid whereas propane is a gas. Explain.

1 point is earned for recognizing that methanol has hydrogen-bonding IMF’s and propane has LDF’s.

1 point is earned for recognizing that hydrogen-bonding results in a greater IMF causing methanol to be a liquid.

Propane molecules are nonpolar and only interact with one another via London dispersion forces. Methanol molecules are polar and hydrogen bonds (as well as London forces) can form among them. Because hydrogen bonds are stronger than London forces, methanol has greater intermolecular attractions. Thus it takes a higher temperature for methanol molecules to have enough kinetic energy (on average) to overcome their stronger intermolecular attractions, thus methanol has the higher boiling point.

3.

Two types of intermolecular forces present in liquid H2S are London (dispersion) forces and dipole-dipole forces.

a) Compare the strength of the London (dispersion) forces in liquid H2S to the strength of the London (dispersion) forces in liquid H2O. Explain.

1 point is earned for the correct answer with a correct explanation.

The strength of the London forces in liquid H2S is greater than that of the London forces in liquid H2O. The electron cloud of H2S has more electrons and is thus more polarizable than the electron cloud of the H2O molecule.

b) Compare the strength of the dipole-dipole forces in liquid H2S to the strength of the dipole-dipole forces in liquid H2O. Explain.

1 point is earned for the correct answer with a correct explanation.

The strength of the dipole-dipole forces in liquid H2S is weaker than that of the dipole-dipole forces in liquid H2O. The net dipole moment of the H2S molecule is less than that of the H2O molecule. This results from the lesser polarity of the H–S bond compared with that of the H–O bond (S is less electronegative than O).

AP Chemistry


4. Propanoic acid, C2H5COOH has a lower boiling point than butanoic acid, C3H7COOH.

a) Identify all the types of intermolecular forces present among the molecules in propanoic acid.

1 point is earned for identifying both London dispersion forces and hydrogen bonding.

London dispersion forces, dipole-dipole forces, and hydrogen bonding. (Identifying dipole-dipole forces is not required to earn the point.)

b) Which of the types of intermolecular forces that you identified in part a) is most responsible for the difference in boiling points of the two acids?

1 point is earned for the correct answer. London dispersion forces.

5. Br2 + Cl2 → 2 BrCl

Br2 and Cl2 can react to form the compound BrCl .

The boiling point of Br2 is 332K, whereas the boiling point of BrCl is 278K. Explain this difference in boiling point in terms of all the intermolecular forces present between molecules of each substance.

The response meets both of the following criteria.

The response indicates that the only intermolecular attractions in Br2(l) are London forces, while those in BrCl(l) include both London forces and dipole-dipole forces.

The response indicates that due to the greater polarizability of the electron cloud of Br2(l)

compared to that of BrCl(l) , the London forces in Br2(l) are stronger than the combined intermolecular forces in BrCl(l) and thus the boiling point of Br2(l) is greater than that of BrCl(l) .

AP Chemistry


6. Answer the following questions by using principles of molecular structure and intermolecular forces.

a) Structures of the pyridine molecule and the benzene molecule are shown below.

Pyridine is soluble in water, whereas benzene is not soluble in water. Account for the difference in solubility. You must discuss both of the substances in your answer.

1 point is earned for identifying a relevant structural difference between pyridine and benzene.

1 point is earned for indicating that pyridine is soluble in water because pyridine can form strong dipole-dipole interactions (or hydrogen bonds) with water, while benzene cannot.

Pyridine is polar (and capable of forming hydrogen bonds with water), while the nonpolar benzene is not capable of forming hydrogen bonds. Pyridine will dissolve in water because of the strong hydrogen bonds (or dipole-dipole intermolecular interactions) that exist between the lone pair of electrons on pyridine’s nitrogen atom and the solvent water molecules. No such strong intermolecular interaction can exist between benzene and water, so benzene is insoluble in water.

AP Chemistry


Q6 continued

b) Structures of the dimethyl ether molecule and the ethanol molecule are shown below.

The normal boiling point of dimethyl ether is 250 K, whereas the normal boiling point of ethanol is 351 K. Account for the difference in boiling points. You must discuss both of the substances in your answer.

1 point is earned for recognizing that ethanol molecules can form intermolecular hydrogen bonds, whereas dimethyl ether molecules do not form intermolecular hydrogen bonds.

1 point is earned for recognizing that, compared to the energy required to overcome the weaker intermolecular forces in liquid dimethyl ether, more energy is required to overcome the stronger hydrogen bonds in liquid ethanol, leading to a higher boiling point.

The intermolecular forces of attraction among molecules of dimethyl ether consist of London (dispersion) forces and weak dipole-dipole interactions. In addition to London forces and dipole-dipole interactions that are comparable in strength to those in dimethyl ether, ethanol can form hydrogen bonds between the H of one molecule and the O of a nearby ethanol molecule. Hydrogen bonds are particularly strong intermolecular forces, so they require more energy to overcome during the boiling process. As a result, a higher temperature is needed to boil ethanol than is needed to boil dimethyl ether.

c) The normal boiling point of Cl2 (l) (238 K) is higher than the normal boiling point of HCl(l) (188 K). Account for the difference in normal boiling points based on the types of intermolecular forces in the substances. You must discuss both of the substances in your answer.

1 point is earned for recognizing that the London forces among Cl2 molecules must be larger than the intermolecular forces (London and dipole-dipole) among HCl molecules.

1 point is earned for recognizing that the strength of the London forces among molecules is proportional to the total number of electrons in each molecule.

The intermolecular forces in liquid Cl2 are London (dispersion) forces, whereas the intermolecular forces in liquid HCl consist of London forces and dipole-dipole interactions. Since the boiling point of Cl2 is higher than the boiling point of HCl, the London forces among Cl2 molecules must be greater than the London and dipole-dipole forces among HCl molecules. The greater strength of the London forces between Cl2

molecules occurs because Cl2 has more electrons than HCl, and the strength of the London interaction is proportional to the total number of electrons.

AP Chemistry


3.2 MC Questions1.

The ionic compounds NaCl and MgS are represented by the diagrams above. Which statement correctly identifies diagram 1 and identifies the compound with the lower melting point, explaining why?

A Diagram 1 represents NaCl ; it has a lower melting point than MgS has because the coulombic attractions between the singly charged Na+ ions and the Cl- ions in NaCl are stronger than those between the ions in MgS.

B Diagram 1 represents NaCl ; it has a lower melting point than MgS because the coulombic attractions between its singly charged Na+ ions and the Cl- ions are weaker than those between the ions in MgS.

C Diagram 1 represents MgS ; it has a lower melting point than NaCl because the coulombic attractions between its doubly charged Mg2+ ions and the S2- ions are stronger than those between the ions in NaCl.

D Diagram 1 represents MgS ; it has a lower melting point than NaCl because the coulombic attractions between the doubly charged Mg2+ ions and the S2_ ions are weaker than those between the ions in NaCl.

2. A sample of a hard, solid binary compound at room temperature did not conduct electricity as a pure solid but became highly conductive when dissolved in water.

Which of the following types of interactions is most likely found between the particles in the substance?

A ionic bonds B metallic bonds

C covalent bonds D hydrogen bonds

O

O

AP Chemistry


3. A student is given a sample of a pure, white crystalline substance.

Which of the following would be most useful in providing data to determine if the substance is an ionic compound?

A Examining the crystals of the substance under a microscope

B Determining the density of the substance

C Testing the electrical conductivity of the crystals

D Testing the electrical conductivity of an aqueous solution of the substance

4.

Based on the diagram above, which of the following best helps to explain why MgO(s) is not able to conduct electricity, but MgO(l) is a good conductor of electricity?

A MgO(s) does not contain free electrons, but MgO(l) contains free electrons that can flow.

B MgO(s) contains no water, but MgO(l) contains water that can conduct electricity.

C MgO(s) consists of separate Mg2+ ions and O2− ions, but MgO(l) contains MgO molecules that can conduct electricity.

D MgO(s) consists of separate Mg2+ ions and O2− ions held in a fixed lattice, but in MgO(l) the ions are free to move and conduct electricity.

5. Which of the following could be the identity of a white crystalline solid that exhibits the following properties?

• It melts at 320°C.• It does not conduct electricity as a solid.• It conducts electricity in an aqueous solution.

A C6H12O6(s)

B NaOH(s)

C SiO2(s)

D Cu(s)

O

O

O

AP Chemistry


6.

The table above provides some information about two types of steel, both of which are alloys of iron and carbon. Which of the following best helps to explain why high-carbon steel is more rigid than low-carbon steel?

A Elemental carbon is harder than elemental iron.

B The additional carbon atoms within the alloy make the high-carbon steel less dense.

C The additional carbon atoms within the alloy increase the thermal conductivity of the high-carbon steel.

D The additional carbon atoms within the alloy make it more difficult for the iron atoms to slide past one another.

7. Which statement best helps to explain the observation that NH3 (l) boils at -28°C , whereas PH3 (l) boils at -126 °C ?

A The dispersion forces in NH3 are weaker than the dispersion forces in PH3.

B The dispersion forces in NH3 are stronger than the dipole-dipole forces in PH3.

C NH3 has hydrogen bonding that is stronger than the dipole-dipole forces in PH3.

D NH3 has hydrogen bonding that is weaker than the dipole-dipole forces in PH3.

8. A certain crystalline substance that has a low melting point does not conduct electricity in solution or when melted. This substance is likely to be

A a covalent network solid B a metallic solid

C a polymer D an ionic solid E a molecular solid

9. At room temperature I2(s) is a molecular solid. Which of the following provides a characteristic of I2(s) with a correct explanation?

A It has a high melting point because it has weak intermolecular forces.

B It is hard because it forms a three dimensional covalent network.

C It is not a good conductor of electricity because its valence electrons are localized in bonding and nonbonding pairs.

D It is very soluble in water because its molecules are polar.

O

O

O

O

AP Chemistry


10.

The structure of one form of boron nitride is represented above.

This form of boron nitride is one of the hardest substances known. Which of the following best helps explain why boron nitride is so hard?

A Boron ions and nitrogen ions are held together by ionic bonds.

B Boron nitride is a network solid of atoms connected by covalent bonds with fixed bond angles.

C Boron nitride is an alloy, and alloys are typically harder than the elements used to make them.

D Boron nitride is a polymer made of long chains of boron atoms and nitrogen atoms held together by london dispersion forces.

11. In solid methane, the forces between neighboring CH4 molecules are best characterized as

A ionic bonds B covalent bonds C hydrogen bonds

D ion-dipole forces E london (dispersion) forces

12. The best explanation for the fact that diamond is extremely hard is that diamond crystals

A are made up of atoms that are intrinsically hard because of their electronic structures

B consist of positive and negative ions that are strongly attracted to each other

C are giant molecules in which each atom forms strong covalent bonds with all of its neighboring atoms

D are formed under extreme conditions of temperature and pressure

E contain orbitals or bands of delocalized electrons that belong not to single atoms but to each crystal as a whole

O

O

O

AP Chemistry


3.2 FRQ1. The structures for glucose, C6H12O6 , and cyclohexane, C6H12 , are shown opposite.

a) Identify the type(s) of intermolecular attractive forces in

i) pure glucose

1 point is earned for the correct answer.

Hydrogen bonding OR dipole-dipole interactions OR van der Waals interactions (London dispersion forces may also be mentioned.)

ii) pure cyclohexane

1 point is earned for London dispersion forces.

b) Glucose is soluble in water but cyclohexane is not soluble in water. Explain.

1 point is earned for explaining the solubility of glucose in terms of hydrogen bonding or dipole-dipole interactions with water.

1 point is earned for explaining the difference in the polarity of cyclohexane and water.

The hydroxyl groups in glucose molecules can form strong hydrogen bonds with the solvent (water) molecules, so glucose is soluble in water. In contrast, cyclohexane is not capable of forming strong intermolecular attractions with water (no hydrogen bonding), so the water-cyclohexane interactions are not as energetically favorable as the interactions that already exist among polar water molecules.

OR - 1 point is earned for any one of the three concepts; two points are earned for any two of the three concepts.

• Glucose is polar and cyclohexane is nonpolar. • Polar solutes (such as glucose) are generally soluble in polar solvents such as water. • Nonpolar solutes (such as cyclohexane) are not soluble in the polar solvent.

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2. The Lewis electron-dot diagrams for molecules of CH4 and CCl4 are shown opposite.

Which compound has the higher boiling point? Justify your answer. The type(s) of intermolecular force(s) in both substances should be included in your answer.

The response meets both of the criteria below:

The response indicates that both compounds have London dispersion forces. (If the response indicates that either of the compounds has any other type of intermolecular force, the criterion is not met.)

The response indicates that has the higher boiling point and that it has greater intermolecular forces.

AP Chemistry


3.3 FRQ1. The Lewis electron-dot diagrams for molecules of CH4 and CCl4 are shown opposite.

CCl4(l) is placed in a previously evacuated container at 30°C, and some of the CCl4(l) evaporates.

In the box below, draw a particulate diagram to show the species in the container after some of the CCl4(l) has evaporated.

The phases of the species should be indicated by the spacing and distribution of the particles in the diagram. Some of the species shown in the legend will not be used.

The drawing meets all of the criteria below:

There are closely spaced undefined molecules near the bottom of the box. There are more widely spaced undefined molecules in the box. There are no separate undefined molecules, undefined atoms, or undefined atoms in the box.

3.3 MC Question1. Of the following, the best explanation for the fact that most gases are easily compressed is that the molecules in a gas

A are in constant motion

B are relatively far apart

C have relatively small masses

D have a real, nonzero volume E move slower as temperature decreases

O

AP Chemistry


3.4 MC Questions 1. The pressure, in atm, exerted by 1.85 mol of an ideal gas placed in a 3.00 L container at 35.0°C is given by which of the following expressions?

A B C

D E

2. The table below contains information about samples of four different gases at 273 K. The samples are in four identical rigid containers numbered 1 through 4.

On the basis of the data provided above, the gas in container 3 could be

A CH4 B O2 C Ar D CO2

3. This question refer to three gases in identical rigid containers under the conditions given in the table below.

The density of the gas, in g/L, is

A greatest in container A B greatest in container B

C greatest in container C D the same for all 3 containers

O

O

O

AP Chemistry


4.

The figure above represents three sealed 1.0 L vessels, each containing a different inert gas at 298 K. The pressure of Ar in the first vessel is 2.0 atm. The ratio of the numbers of Ar, Ne, and He atoms in the vessels is 2 : 1 : 6, respectively.

After all the gases are combined in a previously evacuated 2.0 L vessel, what is the total pressure of the gases at 298 K?

A 3.0 atm B 4.5 atm C 9.0 atm D 18 atm

5.

The reaction between NO(g) and O2(g) to produce NO2(g) in a rigid reaction vessel is represented in the diagram above. The pressure inside the container is recorded using a pressure gauge.

Which of the following statements correctly predicts the change in pressure as the reaction goes to completion at constant temperature, and provides the correct explanation?

A The pressure will increase because the product molecules have a greater mass than either of the reactant molecules.

B The pressure will decrease because there are fewer molecules of product than of reactants.

C The pressure will decrease because the product molecules have a lower average speed than the reactant molecules.

D The pressure will not change because the total mass of the product molecules is the same as the total mass of the reactant molecules.

O

O

AP Chemistry


6. When a sample of oxygen gas in a closed con tainer of constant volume is heated until its abso lute tem pera ture is doubled, which of the follow ing is also dou bled?

A The density of the gas

B The pressure of the gas

C The average velocity of the gas molecules

D The number of molecules per cm3

E The potential energy of the molecules

7. Given that the density of Hg(l) at 0°C is about 14 g mL- 1, which of the following is closest to the volume of one mole of Hg(l) at this temperature?

A 0.070 mL B 0.14 mL C 1.4 mL D 14 mL E 28 mL

8. A sample of an unknown gas from a cylinder is collected over water in the apparatus shown opposite.

After all the gas sample has been collected, the water levels inside and outside the gas collection tube are made the same.

Measurements that must be made to calculate the molar mass of the gas include all of the following except

A atmospheric pressure.

B temperature of the water

C volume of gas in the gas-collection tube

D initial and final mass of the gas cylinder

E mass of the water in the apparatus

O

O

O

AP Chemistry


The following two questions refer to the 1 L flasks shown below.

9. Which flask contains the sample with the greatest density?

A flask A B flask B C flask C D flask D E flask E

10. Which flask contains the smallest number of moles of gas?

A flask A B flask B C flask C D flask D E flask E

11.

Diagram 1 above shows equimolar samples of two gases inside a container fitted with a removable barrier placed so that each gas occupies the same volume.

The barrier is carefully removed as the temperature is held constant. Diagram 2 above shows the gases soon after the barrier is removed. Which statement describes the changes to the initial pressure of each gas and the final partial pressure of each gas in the mixture and also indicates the final total pressure?

A The partial pressure of each gas in the mixture is double its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases.

B The partial pressure of each gas in the mixture is double its initial pressure; the final total pressure is twice the sum of the initial pressures of the two gases.

C The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases.

D The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is the same as the sum of the initial pressures of the two gases.

O

O

O

AP Chemistry


12. A gas mixture at 0°C and 1.0 atm contains 0.010 mol of H2, 0.015 mol of O2, and 0.025 mol of N2. Assuming ideal behavior, what is the partial pressure of hydrogen gas (H2) in the mixture?

A About 0.010 atm, because there is 0.010mol of H2 in the sample.

B About 0.050 atm, because there is 0.050mol of gases at 0°C and 1.0 atm.

C About 0.20 atm, because H2 comprises 20% of the total number of moles of gas.

D About 0.40 atm, because the mole ratio of H2 : O2 : N2 is 0.4 : 0.6 : 1.

13. An equimolar mixture of N2(g) and Ar(g) is kept inside a rigid container at a constant temperature of 300 K. The initial partial pressure of Ar in the mixture is 0.75atm.

An additional amount of Ar was added to the container, enough to double the number of moles of Ar gas in the mixture. Assuming ideal behavior, what is the final pressure of the gas mixture after the addition of the Ar gas?

A 0.75 atm, because increasing the partial pressure of Ar decreases the partial pressure of N2.

B 1.13 atm, because 33% of the moles of gas are N2.

C 1.50 atm, because the number of moles of N2 did not change.

D 2.25 atm, because doubling the number of moles of Ar doubles its partial pressure.

14. A 2 L sample of N2(g) and a 1 L sample of Ar(g), each originally at 1 atm and 0°C, are combined in a 1 L tank. If the temperature is held constant, what is the total pressure of the gases in the tank?

A 1 atm B 2 atm C 3 atm D 4 atm E 5 atm

15. A flask contains 0.25 mole of SO2(g), 0.50 mole of CH4(g), and 0.50 mole of O2(g). The total pressure of the gases in the flask is 800 mm Hg. What is the partial pressure of the SO2(g) in the flask? (All pressures are mm Hg)

A 800 B 600 C 250 D 200 E 160

16. A sealed vessel contains 0.200 mol of oxygen gas, 0.100 mol of nitrogen gas, and 0.200 mol of argon gas. The total pressure of the gas mixture is 5.00 atm. The partial pressure of the argon is

A 0.200 atm B 0.500 atm C 1.000 atm D 2.000 atm E 5.000 atm

O

O

O

O

O

AP Chemistry


17. Equal masses of He and Ne are placed in a sealed container. What is the partial pressure of He if the total pressure in the container is 6 atm?

A 1 atm B 2 atm C 3 atm D 4 atm E 5 atm

18. When 4.0 L of He(g), 6.0 L of N2(g), and 10. L of Ar(g), all at 0°C and 1.0 atm, are pumped into an evacuated 8.0 L rigid container, the final pressure in the container at 0°C is

A 0.5 atm B 1.0 atm C 2.5 atm D 4.0 atm

19. When 6.0 L of He(g) and 10. L of N2(g), both at 0°C and 1.0 atm, are pumped into an evacuated 4.0 L rigid container, the final pressure in the container at 0°C is

A 2.0 atm B 4.0 atm C 6.4 atm D 8.8 atm D 16 atm

20. Equal masses of three different ideal gases, X, Y, and Z, are mixed in a sealed rigid container. If the tem perature of the system remains constant, which of the following statements about the par tial pres sure of gas X is correct?

A It is equal to ⅓ the total pressure

B It depends on the intermolecular forces of at trac tion between molecules of X, Y, and Z.

C It depends on the relative molecular masses of X, Y, and Z.

D It depends on the average distance traveled be tween molecular collisions.

E It can be calculated with knowledge only of the volume of the container.

O

O

O

O

AP Chemistry


3.4 FRQ

1. CaCO3(s) ⇌ CaO(s) + CO2(g)

When heated strongly, solid calcium carbonate decomposes to produce solid calcium oxide and carbon dioxide gas, as represented by the equation above.

A 2.0mol sample of CaCO3(s) is placed in a rigid 100.L reaction vessel from which all the air has been evacuated. The vessel is heated to 898°C at which time the pressure of CO2(g) in the vessel is constant at 1.00atm, while some CaCO3(s) remains in the vessel.

a) Calculate the number of moles of CO2(g) present in the vessel at equilibrium.

The response includes both the following set up and calculation:

n = PV / RT = (1.00 atm)(100 L) / (0.0821 L.atm.mol-1.K-1)(1171 K) = 1.04 mol

b) The experiment was repeated, but this time starting with a 4.0mol sample of CaCO3(s). On the following graph, draw a curve showing how the pressure of CO2(g) would change over time as the vessel is heated to 898°C and equilibrium is established.

The response shows a graph that starts at the origin, rises to right, and levels off (slope = 0) at 1.00 atm

AP Chemistry


2. CaCO3(s) ⇌ CaO(s) + CO2(g)

When heated, calcium carbonate decomposes according to the equation above. In a study of the decomposition of calcium carbonate, a student added a 50.0 g sample of powdered CaCO3(s) to a 1.00 L rigid container.

The student sealed the container, pumped out all the gases, then heated the container in an oven at 1100 K. As the container was heated, the total pressure of the CO2(g) in the container was measured over time. The data are plotted in the graph opposite.

The student repeated the experiment, but this time the student used a 100.0 g sample of powdered CaCO3(s). In this experiment, the final pressure in the container was 1.04 atm, which was the same final pressure as in the first experiment.

a) Calculate the number of moles of CO2(g) present in the container after 20 minutes of heating.

1 point is earned for the proper setup using the ideal gas law and an answer that is consistent with the setup. PV = nRT

n = PV / RT = (1.04 atm)(1.00 L) / (0.0821 L.atm.mol-1.K-1)(1100 K)

= 0.0115 mol

b) The student claimed that the final pressure in the container in each experiment became constant because all of the CaCO3(s) had decomposed. Based on the data in the experiments, do you agree with this claim? Explain.

1 point is earned for disagreement with the claim and for a correct justification using stoichiometry or a discussion of the creation of an equilibrium condition.

AP Chemistry


3. A student was assigned the task of determining the molar mass of an unknown gas.

The student measured the mass of a sealed 843 mL rigid flask that contained dry air. The student then flushed the flask with the unknown gas, resealed it, and measured the mass again.

Both the air and the unknown gas were at 23.0°C and 750. torr. The data for the experiment are shown in the table below.

a) Calculate the mass, in grams, of the dry air that was in the sealed flask. (The density of dry air is 1.18 g L−1 at 23.0°C and 750. torr.)

1 point is earned for the correct setup and calculation of mass.

m = D x V = (1.18 g L−1)(0.843 L) = 0.995 g

b) Calculate the mass, in grams, of the sealed flask itself (i.e., if it had no air in it).

1 point is earned for subtracting the answer in part a) from 157.70 g.

157.70 g − 0.995 g = 156.71 g

c) Calculate the mass, in grams, of the unknown gas that was added to the sealed flask.

1 point is earned for subtracting the answer in part b) from 158.08 g.

158.08 g − 156.71 g = 1.37 g

d) Using the information above, calculate the value of the molar mass of the unknown gas.

1 point is earned for the conversion of pressure (if necessary) and temperature and the use of the appropriate R.

1 point is earned for the correct setup and calculation of moles of gas.

n = PV / RT = (750/760 atm)(0.843 L) / (0.0821 L.atm.mol-1.K-1)(296 K)

= 0.0342 mol

1 point is earned for the correct setup and calculation of molar mass.

OR - see next page

AP Chemistry


Q3. d) continued

If calculation is done in a single step,

1 point is earned for the correct P and T, 1 point is earned for the correct density, and 1 point is earned for the correct answer.

molar mass = 1.37 g / 0.0342 mol = 40.1 g mol-1

OR molar mass = DRT / P = (1.37g / 0.843L).(0.0821 L.atm.mol-1.K-1)(296K) / (750/760 atm) = 40.0 g mol-1

Note: Significant figures were checked in this problem: parts a) and d) were scored with ±1 significant figure needed, and parts b) and c) were scored with the correct number of significant figures needed for the subtraction.

e) After the experiment was completed, the instructor informed the student that the unknown gas was carbon dioxide (44.0 g mol−1) .

Calculate the percent error in the value of the molar mass calculated in part d).

1 point is earned for the correct setup and answer.

percent error = ( |44.0 g mol-1 - 40.1 g mol-1| / 44.0 g mol-1 ) x 100 = 8.9 %

f) For each of the following two possible occurrences, indicate whether it by itself could have been responsible for the error in the student’s experimental result.

You need not include any calculations with your answer. For each of the possible occurrences, justify your answer.

Occurrence 1: The flask was incompletely flushed with CO2(g) , resulting in some dry air remaining in the flask.

1 point is earned for the correct reasoning and conclusion.

This occurrence could have been responsible. The dry air left in the flask is less dense (or has a lower molar mass) than CO2

gas at the given T and P. This would result in a lower mass of gas in the flask and a lower result for the molar mass of the unknown gas.

AP Chemistry


Q3 contd Occurrence 2: The temperature of the air was 23.0°C, but the temperature of the CO2(g) was lower than the reported 23.0°C.

1 point is earned for the correct reasoning and conclusion.

This occurrence could not have been responsible. The density of CO2 is greater at the lower temperature. A larger mass of CO2 would be in the flask than if the CO2 had been at 23.0°C, resulting in a higher calculated molar mass for the unknown gas.

g) Describe the steps of a laboratory method that the student could use to verify that the volume of the rigid flask is 843 mL at 23.0°C. You need not include any calculations with your answer.

1 point is earned for a valid method.

Valid methods include the following: 1. Find the mass of the empty flask. Fill the flask 1. with a liquid of known density (e.g., water at 23°C),and measure the mass of the liquid-filled flask. Subtract to find the mass of the liquid. Using the known density and mass, calculate the volume. 2. Measure 843 mL of a liquid (e.g., water) in a 1,000 mL graduated cylinder and transfer the liquid quantitatively into the flask to see if the water fills the flask completely.

4. After elemental analysis the empirical formula of a compound was determined to be CH2Br . It was also found to have a vapor density of 6.00 g L−1 at 375 K and 0.983 atm.

Using these data, determine the following.

a) The molar mass of the compound

1 point is earned for applying the gas law (PV = nRT) to calculate n. n = PV / RT = (0.983 atm)(1.00 L) / (0.0821 L.atm.mol-1.K-1)(375 K)

= 0.0319 mol

1 point is earned for calculating the molar mass, = 6.00 g / 0.0139 mol = 188 g mol-1

b) The molecular formula of the compound

1 point is earned for the molecular formula that is consistent with the molar mass calculated in part a).

Each CH2Br unit has mass of 12.011 + 2(1.0079) + 79.90 = 93.9 g, and 188 / 93.9 ≃ 2.00, so there must be two CH2Br units per molecule.

Therefore, the molecular formula of the compound is C2H4Br2 .

AP Chemistry


5. C(s) + CO2(g) ⇌ 2 CO(g)

Solid carbon and carbon dioxide gas at 1,160 K were placed in a rigid 2.00 L container, and the reaction represented above occurred.

As the reaction proceeded, the total pressure in the container was monitored. When equilibrium was reached, there was still some C(s) remaining in the container. Results are recorded in the table opposite.

a) Calculate the number of moles of CO2(g) initially placed in the container. (Assume that the volume of the solid carbon is negligible.)

1 point is earned for correct setup. 1 point is earned for the correct answer.

n = PV / RT = (5.00 atm)(2.00 L) / (0.0821 L.atm.mol-1.K-1)(1,160 K)

= 0.105 mol

b) For the reaction mixture at equilibrium at 1,160 K, the partial pressure of the CO2(g) is 1.63 atm. Calculate the partial pressure of CO(g)

1 point is earned for correct answer supported by correct method.

PCO2 + PCO = Ptotal PCO = Ptotal - PCO2 = 8.37 -1.63 = 6.74 atm

AP Chemistry


3.5 MC Questions1. A 0.5 mol sample of He(g) and a 0.5 mol sample of Ne(g) are placed separately in two 10.0 L rigid containers at 25°C. Each container has a pinhole opening. Which of the gases, He(g) or Ne(g), will escape faster through the pinhole and why?

A He(g) will escape faster because the He(g) atoms are moving at a higher average speed than the Ne(g) atoms.

B Ne(g) will escape faster because its initial pressure in the container is higher.

C Ne(g) will escape faster because the Ne(g) atoms have a higher average kinetic energy than the He(g) atoms.

D Both gases will escape at the same rate because the atoms of both gases have the same average kinetic energy.

2. A rigid metal tank contains oxygen gas. Which of the following applies to the gas in the tank when additional oxygen is added at constant temperature?

A The volume of the gas increases.

B The pressure of the gas decreases.

C The average speed of the gas molecules remains the same.

D The total number of gas molecules remains the same.

E The average distance between the gas molecules increases.

3. A sample of an ideal gas is cooled from 50.0°C to 25.0°C in a sealed container of constant volume. Which of the following values for the gas will decrease?

I. The average molecular mass of the gas II. The average distance between the molecules III. The average speed of the molecules

A I only

B II only

C III only

D I and III

E II and III

O

O

O

AP Chemistry


4. At standard temperature and pressure, a 0.50 mol sample of H2 gas and a separate 1.0 mol sample of O2 gas have the same

A average molecular kinetic energy

B average molecular speed

C volume

D effusion rate

E density

5. The diagram above shows the distribution of speeds for a sample of N2(g) at 25°C.

Which of the following graphs shows the distribution of speeds for a sample of O2(g) at 25°C (dashed line) ?

A B

C D

O

O

AP Chemistry


6. Equal numbers of moles of He(g), Ar(g), and Ne(g) are placed in a glass vessel at room temperature. If the vessel has a pinhole-sized leak, which of the following will be true regarding the relative values of the partial pressures of the gases remaining in the vessel after some of the gas mixture has effused?

A PHe < PNe < PAr

B PHe < PAr < PNe

C PNe < PAr < PHe

D PAr < PHe < PNe

E PHe = PAr = PNe

7. The following question refers to the 1 L flasks shown below.

In which flask do the molecules have the greatest average speed?

A A B B C C D D E E

8. A 1L sample of helium gas at 25°C and 1atm is combined with a 1L sample of neon gas at 25°C and 1atm. The temperature is kept constant. Which of the following statements about combining the gases is correct?

A The average speed of the helium atoms increases when the gases are combined.

B The average speed of the neon atoms increases when the gases are combined.

C The average kinetic energy of the helium atoms increases when the gases are combined.

D The average kinetic energy of the helium atoms and neon atoms do not change when the gases are combined.

O

O

O

AP Chemistry


9.

The graph above shows the distribution of molecular speeds for four different gases at the same temperature. What property of the different gases can be correctly ranked using information from the graph, and why?

A The densities of the gases, because as the density of a gas increases, the average speed of its molecules decreases.

B The pressures of the gases, because the pressure exerted by a gas depends on the average speed with which its molecules are moving

C The volumes of the gases, because at a fixed temperature the volume of a gas can be calculated using the equation PV=nRT .

D The molecular masses of the gases, because the gas molecules have the same average kinetic energy and mass can be calculated using the equation KEavg=½mv2 .

10.

The average kinetic energy of the gas molecules is?

A greatest in A B greatest in B

C greatest in C D the same in all three containers

O

O

AP Chemistry


11.

The experimental apparatus represented above is used to demonstrate the rates at which gases diffuse. When the cotton balls are placed in the ends of a tube at the same time, the gases diffuse from each end and meet somewhere in between, where they react to form a white solid. Which of the following combinations will produce a solid closest to the center of the tube?

A HCl and CH3NH2

B HCl and NH3

C HBr and CH3NH2

D HBr and NH3

12.

The volume of a sample of air in a cylinder with a movable piston is 2.0 L at a pressure P1, as shown in the diagram above. The volume is increased to 5.0 L as the temperature is held constant. The pressure of the air in the cylinder is now P2. What effect do the volume and pressure changes have on the average kinetic energy of the molecules in the sample?

A The average kinetic energy increases.

B The average kinetic energy decreases.

C The average kinetic energy stays the same.

D It cannot be determined how the kinetic energy is affected without knowing P1 and P2.

O

O

AP Chemistry


13.

The distribution of speeds of H2(g) molecules at 273 K and 1 atm is shown in the diagram above. Which of the following best shows the speed distribution of He(g) atoms under the same conditions of temperature and pressure?

A B

C D


Under the conditions given, consider containers 1, 2, and 4 only. The average speed of the gas particles is

A greatest in container 1 B greatest in container 2

C greatest in container 4 D the same in containers 1, 2, and 4

O

O

AP Chemistry


15. Two flexible containers for gases are at the same tem perature and pressure. One holds 0.50 gram of hy drogen and the other holds 8.0 grams of oxy gen. Which of the following statements re garding these gas samples is FALSE?

A The volume of the hydrogen container is the same as the volume of the oxygen container.

B The number of molecules in the hydrogen con tainer is the same as the number of molecules in the oxygen container.

C The density of the hydrogen sample is less than that of the oxygen sample.

D The average kinetic energy of the hydrogen molecules is the same as the average kinetic en ergy of the oxygen molecules.

E The average speed of the hydrogen molecules is the same as the average speed of the oxygen molecules.

16. At 27°C, a rigid 2.0 L vessel is filled with N2(g) and sealed. The initial pressure of N2(g) is 720 mm Hg. The vessels is heated to 127°C and allowed to reach a constant pressure.

The gas particles in the vessel at 27°C are represented in the diagram above. The lengths of the arrows represent the speeds of the particles. Which of the following diagrams best represents the particles when the vessel is heated to 127°C?

A B

C D

O

O

AP Chemistry


17.

The graph above shows the speed distribution of molecules in a sample of a gas at a certain temperature. Which of the following graphs shows the speed distribution of the same molecules at a lower temperature (as a dashed curve) ?

A B

C D

O

AP Chemistry


3.5 FRQ

1. A student is doing experiments with CO2(g). Originally, a sample of the gas is in a rigid container at 299K and 0.70atm. The student increases the temperature of the CO2(g) in the container to 425K. .

a) Describe the effect of raising the temperature on the motion of the CO2(g) molecules.

The response indicates that the average speed of the molecules increases as temperature increases

b) Calculate the pressure of the CO2(g) in the container at 425K.

The response shows a calculation similar to the following.

P1 / T1 = P2 / T2 0.70 atm / 425 K = P2 / 425 K P2 = 0.99 atm

c) In terms of kinetic molecular theory, briefly explain why the pressure of the CO2(g) in the container changes as it is heated to 425K

The response meets one or both of the following criteria. The response indicates that faster-moving gas particles collide more frequently with the walls of the container, thus increasing the pressure. The response indicates that faster-moving gas particles collide more forcefully with the walls of the container, thus increasing the pressure.

* d) The student measures the actual pressure of the CO2(g) in the container at 425K and observes that it is less than the pressure predicted by the ideal gas law. Explain this observation.

The response indicates that the attractive forces between CO2 molecules result in a pressure that is lower than that predicted by the ideal gas law.

* This question might be best left until after next section on deviations fron Ideal Gas.

AP Chemistry


3.6 MC Questions1. Which of the following behaves most like an ideal gas at the conditions indicated?

A H2(g) molecules at 10-3 atm and 200°C

B O2(g) molecules at 20 atm and 200°C

C SO2(g) molecules at 20 atm and 200°C

D NH3(g) molecules at 20 atm and 200°C

E NH3(g) molecules at 20 atm and 300°C

2. Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference?

A The particle volume of Ar is greater than that of Ne

B Ar atoms have more valence electrons than Ne atoms have, so Ar atoms have greater interparticle forces.

C The intermolecular forces between Ne atoms are greater than those between Ar atoms.

D Ar atoms are more attracted to the walls of the container than Ne atoms are.

3. Which of the following gases deviates most from ideal behavior?

A SO2 B Ne C CH4 D N2 E H2

4. When the actual gas volume is greater than the volume predicted by the ideal gas law, the explanation lies in the fact that the ideal gas law does NOT include a factor for molecular

A volume

B mass

C velocity

D attractions

E shape

O

O

O

O

AP Chemistry


5. A vessel contains Ar(g) at a high pressure. Which of the following statements best helps to explain why the measured pressure is significantly greater than the pressure calculated using the ideal gas law?

A The molar mass of Ar is relatively large.

B A significant number of Ar2 molecules form.

C The attractive forces among Ar atoms cause them to collide with the walls of the container with less force.

D The combined volume of the Ar atoms is too large to be negligible compared with the total volume of the container.

6. At constant temperature, the behavior of a sample of a real gas more closely approximates that of an ideal gas as its volume is increased because the

A collisions with the walls of the container become less frequent

B average molecular speed decreases

C molecules have expanded

D average distance between molecules becomes greater.

E average molecular kinetic energy decreases.

7. The diagrams opposite represent two samples of Xe gas in containers of equal volume at 280K.

Which of the following correctly compares the two samples in terms of their deviation from ideal gas behavior and explains why?

A The gas in sample 1 would deviate more from ideal behavior because the average distance an Xe atom travels before colliding with another Xe atom is greater.

B The gas in sample 2 would deviate more from ideal behavior because the Xe atoms are closer together, leading to an increase in intermolecular attractions.

C The gas in sample 2 would deviate more from ideal behavior because the average speed of the Xe atoms is less, leading to an increase in intermolecular attractions.

D The gases in both sample 1 and sample 2 would show the same deviation from ideal behavior.

O

O

O

AP Chemistry


8. Which of the following best helps explain why the pressure of a sample of CH4(g) (molar mass 16g/mol) is closer to the pressure predicted by the ideal gas law than a sample of NH3(g) (molar mass 17g/mol) ?

A NH3 molecules are polar while CH4 molecules are not, and the greater attractions between NH3 molecules cause the molecules to collide with the walls of the container with less force.

B NH3 molecules have a greater molar mass than CH4 molecules, so the NH3 molecules collide with the walls of the container with more force.

C CH4 molecules have more hydrogen atoms than NH3 molecules, so CH4 molecules have more hydrogen bonding and greater intermolecular forces.

D CH4 molecules are larger than NH3 molecules, so the actual CH4 molecules take up a significant portion of the volume of the gas.

9. Under which of the following conditions of temperature and pressure would 1.0 mol of the real gas CO2(g) behave most like an ideal gas?

Temperature (K) Pressure (atm) A 100 0.1

B 100 100

C 800 0.1

D 800 100

10. The ideal gas law best describes the properties of which of the following gases at 0°C and 1 atm?

A PH3 B HBr C SO2 D N2

11. Under which of the following conditions of temperature and pressure will H2 gas be expected to behave most like an ideal gas?

A 50 K and 0.10 atm B 50 K and 5.0 atm

C 500 K and 0.10 atm D 500 K and 50 atm

O

O

O

O

AP Chemistry



The best explanation for the lower pressure in container 4 is that SO2 molecules

A have a larger average speed than the other three gases

B occupy a larger portion of the container volume than the other three gases

C have stronger intermolecular attractions than the other three gases

D contain π bonds, while the other gases contain only σ bondsO

AP Chemistry


3.6 FRQ

1. At 400K CH4(g) and CCl4(g)are gases. At this temperature, which compound, CH4(g) or CCl4(g), behaves more like an ideal gas? Justify your answer, including reasoning about both molecules. .

The response meets both of the criteria below:

The response indicates that both compounds have London dispersion forces. (If the response indicates that either of the compounds has any other type of intermolecular force, the criterion is not met.)

The response indicates that CCl4has the higher boiling point and that it has greater intermolecular forces.

2. * d) The student measures the actual pressure of the CO2(g) in the container at 425K and observes that it is less than the pressure predicted by the ideal gas law. Explain this observation.

The response indicates that the attractive forces between CO2 molecules result in a pressure that is lower than that predicted by the ideal gas law.

* Last part of question asked in previous section.

AP Chemistry


3.7 MC Questions1. A 0.20 mol sample of MgCl2(s)and a 0.10 mol sample of KCl(s)are dissolved in water and diluted to 500 mL. What is the concentration of Cl- in the solution?

A 0.15 M B 0.30 M C 0.50 M D 0.60 M E 1.00 M

2. A 360 mg sample of aspirin, C9H8O4, (molar mass 180 g), is dissolved in enough water to produce 200 mL of solution. What is the molarity of aspirin in a 50 mL sample of this solution?

A 0.080 M B 0.040 M C 0.020 M D 0.010 M E 0.0025 M

3. A 40.0 mL sample of 0.25 M KOH is added to 60.0 mL of 0.15 M Ba(OH)2. What is the molar concentration of OH-

(aq) in the resulting solution? (Assume that the volumes are additive.)

A 0.10 M B 0.19 M C 0.28 M D 0.40 M E 0.55 M

4. A student prepares a solution by dissolving 60.00 g of glucose (molar mass 180.2 g mol-1) in enough distilled water to make 250.0 mL of solution. The molarity of the solution should be reported as

A 12.01 M B 12.0 M C 1.332 M D 1.33 M E 1.3 M

5. A student wishes to prepare 2.00 liters of 0.100 mo lar KIO3 (molecular weight 214). The proper proce dure is to weigh out

A 42.8 grams of KIO3 and add 2.00 kilograms of H2O

B 42.8 grams of KIO3 and add H2O until the fi nal homogeneous solution has a volume of 2.00 liters

C 21.4 grams of KIO3 and add H2O until the fi nal homogeneous solution has a volume of 2.00 liters

D 42.8 grams of KIO3 and add 2.00 liters of H2O

E 21.4 grams of KIO3 and add 2.00 liters of H2O

6. Approximately what mass of CuSO4 • 5H2O (250 g mol-1) is required to prepare 250 mL of 0.10 M copper(II) sulfate solution?

A 4.0 g B 6.2 g C 34 g D 85 g E 140 g

O

O

O

O

O

O

AP Chemistry


7. A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration.

Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration.

The original solution used to make the solutions for the standard curve was prepared by dissolving 2.60g of CoCl2 (molar mass 130 g/mol) in enough water to make 100 mL of solution. What is the molar concentration of the solution?

A 0.200 M B 0.500 M C 1.00 M D 5.00 M

8. For an experiment, a student needs 100.0 mL of 0.4220 M NaCl. If the student starts with NaCl(s) and distilled water, which of the following pieces of laboratory glassware should the student use to prepare the solution with the greatest accuracy?

A 25 mL volumetric pipet

B 100 mL Erlenmeyer flask

C 100 mL graduated cylinder

D 100 mL volumetric flask

E 1 L beaker

9. How many mL of 10.0 M HCl are needed to prepare 500 mL of 2.00 M HCl ?

A 1.00 mL B 10.0 mL C 20.0 mL D 100 mL E 200 mL

10. If 200. mL of 0.60 M MgCl2(aq) is added to 400. mL of distilled water, what is the concentration of Mg2+

(aq) in the resulting solution? (Assume volumes are additive).

A 0.20 M B 0.30 M C 0.40 M D 0.60 M E 1.2 M

11. If 50. mL of 1.0 M NaOH is diluted with distilled water to a volume of 2.0 L, the concentration of the resulting solution is

A 0.025 M B 0.050 M C 0.10 M D 0.50 M E 1.0 M

12. How many moles of Na+ ions are in 100.mL of 0.100M Na3PO4(aq) ?

A 0.300 mol B 0.100 mol C 0.0300 mol D 0.0100 mol

O

O

O

O

O

O

AP Chemistry


13. How many grams of CaCl2 (molar mass =111g/mol) are needed to prepare 100 mL of 0.100MCl−

(aq) ions?

A 0.555 g B 1.11 g C 2.22 g D 5.55 g

14. Solid Al(NO3)3 is added to distilled water to produce a solution in which the concentration of nitrate, [NO3

-], is 0.10 M. What is the concentration of aluminum ion, [Al3+], in this solution?

A 0.010 M B 0.033 M C 0.066 M D 0.10 M E 0.30 M

15. The volume of distilled water that should be added to 10.0 mL of 6.00 M HCl(aq) in order to prepare a 0.500 M HCl(aq) solution is approximately

A 50.0 mL B 60.0 mL C 100 mL D 110 mL E 120 mL

16. The volume of water that must be added in order to dilute 40 mL of 9.0 M HCl to a concentration of 6.0 M is closest to

A 10 mL B 20 mL C 30 mL D 40 mL E 60 mL

17. What is the molarity of I-(aq) in a solution that contains 34 g of SrI2 (molar mass 341 g) in

1.0 L of the solution?

A 0.034 M B 0.068 M C 0.10 M D 0.20 M E 0.68 M

18. When 70 milliliter of 3.0 molar Na2CO3 is added to 30 milliliters of 1.0-molar NaHCO3 the result ing concentration of Na+ is

A 2.0 M B 2.4 M C 4.0 M D 4.5 M E 7.0 M

19. Which of the following is the best piece of laboratory glassware for preparing 500.0 mL of an aqueous solution of a solid?

A Volumetric Falsk

B Erlenmeyer flask

C Test tube

D Graduated beaker

E Graduated cylinder

O

O

O

O

O

O

O

AP Chemistry


3.7 FRQ1. A student has 100. mL of 0.400 M CuSO4(aq) and is asked to make 100. mL of 0.150 M CuSO4(aq) for a spectrophotometry experiment.

The following laboratory equipment is available for preparing the solution: centigram balance, weighing paper, funnel, 10 mL beaker, 150 mL beaker, 50 mL graduated cylinder, 100 mL volumetric flask, 50 mL buret, and distilled water.

a) Calculate the volume of 0.400 M CuSO4(aq) required for the preparation.

1 point is earned for the correct volume. M1V1 = M2V2 V2 = (0.150M)(0.100L) / 0.400M V2 = 0.0375 L x (1000 mL/1L) = 37.5 mL

b) Briefly describe the essential steps to most accurately prepare the 0.150 M CuSO4(aq) from the 0.400 M CuSO4(aq) using the equipment listed above.

1 point is earned for using the buret to measure 37.5 mL of 0.400 M CuSO4 solution. 1 point is earned for adding the CuSO4 solution to the volumetric flask and filling to the mark with distilled water.

The student plans to conduct a spectrophotometric analysis to determine the concentration of Cu2+

(aq) in a solution.

The solution has a small amount of Co(NO3)2(aq) present as a contaminant. The student is given the diagram opposite, which shows the absorbance curves for aqueous solutions of Co2+

(aq) and Cu2+(aq).

c) The spectrophotometer available to the student has a wavelength range of 400 nm to 700 nm.

What wavelength should the student use to minimize the interference from the presence of the Co2+

(aq) ions?

1 point is earned for a correct wavelength. 700 nm (Any wavelength from 650 to 700 nm is acceptable.)

AP Chemistry


2. A student is given a sample of CuSO4(s) that contains a solid impurity that is soluble and colorless.

The student wants to determine the amount of CuSO4 in the sample and decides to use a spectrophotometer.

First, the student prepares a calibration graphby measuring the absorbances of CuSO4(aq) solutions of known concentrations. The graph is shown.

a) The student dissolves the entire impure sample of CuSO4(s) in enough distilled water to make 100.mL of solution. Then the student measures the absorbance of the solution and observes that it is 0.30.

Determine the concentration of CuSO4(aq) in the solution.

The response indicates that the absorbance is 0.15 M. (Unit not required. Any value from 0.14 M to 0.16 M is acceptable.)

b) Calculate the number of moles of CuSO4 that were in the impure sample of CuSO4(s)

The response gives the following calculation (or an equivalent): Unit not required.

0.15 mol / 1000 mL x 100. ml = 0.015 mol

c) In addition to the number of moles of CuSO4 calculated in part b), what other quantity must be measured in order to calculate the mass percentage of CuSO4 in the impure sample of CuSO4(s) ?

The response indicates that the mass of the sample must be measured.

AP Chemistry


3. Mg(s) + 2 H+

(aq) → Mg2+(aq) + H2(g)

A student performs an experiment to determine the volume of hydrogen gas produced when a given mass of magnesium reacts with excess HCl(aq) , as represented by the net ionic equation above.

The student begins with a 0.0360 g sample of pure magnesium and a solution of 2.0 M HCl(aq).

a) Calculate the number of moles of magnesium in the 0.0360 g sample.

1 point is earned for the correct number of moles of Mg.

1 mol Mg / 24.30 g Mg x 0.0360 g = 0.00148 mol Mg

b) Calculate the number of moles of HCl(aq) needed to react completely with the sample of magnesium.

1 point is earned for the correct number of moles of HCl.

0.00148 mol Mg x 2 mol H+ / 1 mol Mg = 0.00296 mol HCl

As the magnesium reacts, the hydrogen gas produced is collected by water displacement at 23.0°C. The pressure of the gas in the collection tube is measured to be 749 torr.

c) Given that the equilibrium vapor pressure of water is 21 torr at 23.0°C, calculate the pressure that the H2(g) produced in the reaction would have if it were dry.

1 point is earned for the correct pressure.

P = 749 torr − 21 torr = 728 torr

d) Calculate the volume, in liters measured at the conditions in the laboratory, that the H2(g) produced in the reaction would have if it were dry.

PV = nRT V = nRT/ P

P = 728 torr x 1 atm / 760 torr = 0.958 atm

V = (0.00148 mol H2) (0.0821 L atm mol-1 K-1)(296 K) / 0.958 atm = 0.0375 L

1 point is earned for the correct moles of H2(g). 1 point is earned for agreement of units of P (from part c)) and R.

1 point is earned for the correct temperature substitution and calculated volume.

Note: For errors in significant figures or mathematics, there is a 1 point deduction; For the entire question, only 1 point total can be deducted for each type of error.

AP Chemistry


3.8 & 3.10 MC Questions1.

Based on concepts of polarity and hydrogen bonding, which of the following sequences correctly lists the compounds above in the order of their increasing solubility in water?

A Z < Y < X B Y < Z < X C Y < X < Z D X < Z < Y E X < Y < Z

2.

Benzene, C6H6 , has the structure shown above. Considering the observation that benzene is only sparingly soluble in water, which of the following best describes the intermolecular forces of attraction between water and benzene?

A Benzene is nonpolar, therefore there are no forces between water and benzene.

B The H atoms in benzene form hydrogen bonds with the O atoms in water.

C Benzene is hydrophobic, therefore there is a net repulsion between water and benzene.

D There are dipole-induced dipole and London dispersion interactions between water and benzene.

3. On the basis of molecular structure and bond polarity, which of the following compounds is most likely to have the greatest solubility in water?

A CH4 B CCl4 C NH3 D PH3

4. Sodium chloride is LEAST soluble in which of the following liquids?

A H2O B CCl4 C HF D CH3OH E CH3COOH

O

O

O

O

AP Chemistry


5.

Gases generated in a chemical reaction are sometimes collected by the displacement of water, as shown above. Which of the following gases can be quantitatively collected by this method?

A H2 B CO2 C HCl D SO3OH E CH3COOH

6. M+ is an unknown metal cation with a +1 charge.

A student dissolves the chloride of the unknown metal, MCl, in enough water to make 100.0 mL of solution. The student then mixes the solution with excess AgNO3 solution, causing AgCl to precipitate. The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass ofAgCl is 143 g/mol.)

Mass of unknown chloride MCl = 0.74 g Mass of filter paper = 0.80 g Mass of filter paper plus AgCl precipitate = 2.23 g

Which of the following diagrams best represents the AgNO3 solution before the reaction occurs? Note: water molecules are represented by the symbol

A B

C D

O

O

AP Chemistry


7. The survival of aquatic organisms depends on the small amount of O2 that dissolves in H2O. The diagrams below represent possible models to explain this phenomenon.

Which diagram provides the better particle representation for the solubility of O2 in H2O, and why?

A Diagram 1, because O2 molecules can form hydrogen bonds with the H2O molecules.

B Diagram 1, because O2 and H2O are polar molecules that can interact through dipole-dipole forces.

C Diagram 2, because the polar H2O molecules can induce temporary dipoles on the electron clouds of O2 molecules.

D Diagram 2, because the nonpolar O2 molecules can induce temporary dipoles on the electron clouds of H2O molecules.

8. Of the following organic compounds, which is LEAST soluble in water at 298 K?

A CH3OH, methanol

B CH3CH2CH2OH, l-propanol

C C6H14, hexane

D C6H12O6, glucose

E CH3COOH, ethanoic (acetic) acid

9. Which of the following molecules is least soluble in water?

A B C D

O

O

O

AP Chemistry


10. Based on their Lewis diagrams, which of the following pairs of liquids are most soluble in each other?

A

B

C

D

11. Which of the following substances has the greatest solubility in C5H12(l) at 1 atm?

A SiO2(s)

B NaCl(s)

C H2O(l)

D CCl4(l)

E NH3(g)

O

O

AP Chemistry


12. A student places a piece of I2(s) in 50.0 mL of H2O(l), another piece of I2(s) of the same mass in 50.0 mL of C6H14(l), and shakes the mixtures. The results are shown above. What do the results indicate about the intermolecular interactions of the substances?

A I2 and H2O have similar intermolecular interactions, and I2 and C6H14 do not.

B I2 and C6H14 have similar intermolecular interactions, and I2 and H2O do not.

C I2 , H2O , and C6H14 all have similar intermolecular interactions.

D I2 , H2O , and C6H14 have three completely different types of intermolecular interactions.

13. Ag+(aq) + Cl—

(aq) ⇄ AgCl(s)

A student mixes dilute AgNO3(aq) with excess NaCl(aq) to form AgCl(s), as represented by the net ionic equation above. Which of the diagrams below best represents the ions that are present in significant concentrations in the solution? (Ksp for AgCl is 1.8 x 10−10.)

A B C D

14. Which of the following compounds the LEAST soluble in water

A CH3CH2CH2CH3 B CH3CH2CH2OH C CH3COCH3

D CH3COOH E CH3CH2CH2NH2

O

O

O

AP Chemistry


3.8 & 3.10 FRQ1. A student is given 50.0mL of a solution of Na2CO3 of unknown concentration.

To determine the concentration of the solution, the student mixes the solution with excess 1.0MCa(NO3)2(aq), causing a precipitate to form. The balanced equation for the reaction is shown below.

Na2CO3(aq) + Ca(NO3)2(aq) → 2NaNO3(aq) + CaCO3(s)

a) Write the net ionic equation for the reaction that occurs when the solutions of Na2CO3 and Ca(NO3)2 are mixed.

Ca2+(aq) + CO3

2-(aq) → CaCO3 (s) phase designations not required

b) The diagram below is incomplete. Draw in the species needed to accurately represent the major ionic species remaining in the solution after the reaction has been completed.

The diagram shows one Ca 2+ ion (or more than one Ca2+ ion and the correct number of additional NO3

- ions to balance charge).

AP Chemistry


2. A student prepares a calibration graph by measuring the absorbances of CuSO4(aq) solutions of known concentrations.

A particulate drawing of a single Cu2+ ion is shown in the following box. Draw two H2O molecules in the box to show the most likely orientation of H2O(l) molecules around the Cu2+ ion.

The drawing shows two water molecules generally oriented so the large circle (indicating oxygen) is closer to the ion than the two small circles.

3. The structures of a water molecule and a crystal of LiCl(s) are represented opposite. A student prepares a 1.0 M solution by dissolving 4.2 g of LiCl(s) in enough water to make 100 mL of solution.

In the space provided below, show the interactions of the components of LiCl(aq) by making a drawing that represents the different particles present in the solution. Base the particles in your drawing on the particles shown in the representations above. Include only one formula unit of LiCl and no more than ten molecules of water.

Your drawing must include the following details. • identity of ions (symbol and charge) • the arrangement and proper orientation of the particles in the solution

The sketch should clearly show: 1. a clear representation of at least one Li+ ion and one Cl– ion separated from each other, labeled, and charged;

(answer continues on next page)

AP Chemistry


4.

The compound urea, H2NCONH2 , is widely used in chemical fertilizers. The complete Lewis electron-dot diagram for the urea molecule is shown above.

Urea has a high solubility in water, due in part to its ability to form hydrogen bonds. A urea molecule and four water molecules are represented in the box below.

Draw ONE dashed line (----) to indicate a possible location of a hydrogen bond between a water molecule and the urea molecule.

There is a dashed line connecting a hydrogen atom in water to a nitrogen or oxygen atom in urea or a dashed line connecting an oxygen atom in water to a hydrogen atom in urea.

The sketch should clearly show:

2. each ion surrounded by at least two H2O molecules; and

3. H2O molecules with the proper orientation around each ion (i.e., the oxygen end of the watermolecules closer to the lithium ion and the hydrogen end of the water molecules closer to the chloride ion).

AP Chemistry


3.9 MC Questions1. Which of the following techniques is most appropriate for the recovery of solid KNO3 from an aqueous solution of KNO3?

A Paper chromatography B Filtration C Titration

D Electrolysis E Evaporation to dryness

2.

On the basis of the solubility curves shown above, the greatest percentage of which compound can be recovered by cooling a saturated solution of that compound from 90°C to 30°C?

A NaCl B KNO3 C K2Cr2O7 D K2SO4 E Ce2(SO4)3

O

O

AP Chemistry


3.

The diagram above shows a thin-layer chromatogram of a mixture of products from a chemical reaction. The separation was performed using 50% ethyl acetate in hexane as the solvent (mobile phase) and silica gel as the polar stationary phase.

On the basis of the chromatogram and the information about solvents in the table above, which of the following would be the best way to decrease the distance that the products travel up the plate?

A Use pentane instead of hexane in the solvent.

B Decrease the percentage of ethyl acetate in the solvent.

C Increase the percentage of ethyl acetate in the solvent.

D Add up to 5% methanol to the solvent.

O

AP Chemistry


4. A student obtains a liquid sample of green food coloring that is known to contain a mixture of two solid pigments, one blue and one yellow, dissolved in an aqueous solution of ethanol.

Which of the following laboratory setups is most appropriate for the student to use in order to separate and collect a substantial sample of each of the two pigments?

A B

C D

O

AP Chemistry


5.

A student performed a fractional distillation of a mixture of two straight-chain hydrocarbons, C7H16 and C8H18.

Using four clean, dry flasks, the student collected the distillate over the volume ranges (A, B, C, and D) shown in the graph above.

Over what volume range should the student collect the distillate of the compound with the stronger intermolecular forces?

A A B B C C D D

6. A mixture containing equal numbers of moles of ethyl acetate and butyl acetate was separated using distillation.

Based on the diagrams shown above, which of the following identifies the substance that would be initially present in higher concentration in the distillate and correctly explains why that occurs?

A Ethyl acetate, because it has fewer C-C bonds to break

B Ethyl acetate, because it has a shorter carbon chain and weaker London dispersion forces

C Butyl acetate, because it has more C-C bonds to break

D Butyl acetate, because it has a longer carbon chain and weaker dipole-dipole attractions

O

O

AP Chemistry


7.

In a paper chromatography experiment, a sample of a pigment is separated into two components, X and Y, as shown in the figure above.

The surface of the paper is moderately polar. What can be concluded about X and Y based on the experimental results?

A X has a larger molar mass than Y does.

B Y has a larger molar mass than X does.

C X is more polar than Y.

D Y is more polar than X. O

AP Chemistry


3.11 MC Questions1. Infrared spectroscopy is a useful tool for scientists who want to investigate the structure of certain molecules. Which of the following best explains what can occur as the result of a molecule absorbing a photon of infrared radiation?

A The energies of infrared photons are in the same range as the energies associated with changes between different electronic energy states in atoms and molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of electronic transitions within the molecules.

B The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules.

C The energies of infrared photons are in the same range as the energies associated with different rotational states of molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of transition between different rotational energy states of the molecules.

D The energies of infrared photons are in the same range as the total bond energies of bonds within molecules. Chemical bonds can be completely broken as they absorb infrared photons of characteristic wavelengths, thus revealing the energies of the bonds within the molecules.

2. Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region?

A Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states.

B Microwave photons cause electrons in the molecules to increase their electronic energy states, whereas infrared photons cause the molecules to increase their rotational energy states.

C Microwave photons cause the molecules to increase their vibrational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

D Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

O

O

AP Chemistry


3. N2 molecules absorb ultraviolet light but not visible light. I2 molecules absorb both visible and ultraviolet light. Which of the following statements explains the observations?

A More energy is required to make N2 molecules vibrate than is required to make I2 molecules vibrate.

B More energy is required to remove an electron from an I2 molecule than is required to remove an electron from a N2 molecule.

C Visible light does not produce transitions between electronic energy levels in the N2 molecule but does produce transitions in the I2 molecule..

D The molecular mass of I2 is greater than the molecular mass of N2.

4.

Beta-carotene is an organic compound with an orange color. The diagram above shows the ultraviolet spectrum of beta-carotene. Which of the following statements is true about the absorption bands in the spectrum?

A The absorption band between 250 and 320 nm is due to transitions in electronic energy levels, and the absorption band between 380 and 520 nm is due to transitions in molecular vibrational levels.

B The absorption band between 250 and 320 nm is due to transitions in molecular vibrational levels, and the absorption band between 380 and 520 nm is due to transitions in molecular rotational levels.

C The two main absorption bands are associated with transitions in electronic energy levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

D The two main absorption bands are associated with transitions in molecular vibrational levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

O

O

AP Chemistry


5.

The diagram above represents the absorption spectrum for a pure molecular substance. Which of the following correctly indicates the type of transition observed for the substance in each of the regions of the absorption spectrum?

Region X Region Y Region Z

A molecular vibration molecular rotation electronic transition

B electronic transition molecular rotation molecular vibration

C molecular rotation molecular vibration electronic transition

D electronic transition molecular vibration molecular rotation O

AP Chemistry


3.12 MC Questions The first two questions are based on the information given below.

The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10−19 J

1. Which of the following is closest to the frequency of the light with photon energy of 3.3 × 10−19 J ?

A 5.0 × 10−53 s−1 B 5.0 × 10−16 s−1 C 5.0 × 1014 s−1 D 5.0 × 1052 s−1

2. Using the wavelength information provided above, what is the color of the light?

A Red B Orange C Yellow D Blue



A wavelength of 510nm corresponds to an approximate frequency of 6 × 1014 s−1. What is the approximate energy of one photon of this light?

A 9 × 1047 J B 3 × 1017 J C 5 × 10−7 J D 4 × 10−19 J

O

O

O

AP Chemistry


3.13 MC Questions1.

The diagrams above show the ultraviolet absorption spectra for two compounds.

Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration.

When the student reads the absorbance of the solution at 280nm, the result is too high.

Which of the following is most likely responsible for the error in the measured absorbance?

A The student added too little solute to the acetone before measuring its absorbance. B The student rinsed the cuvette with the solution before filling the cuvette with the solution.

C The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone.

D The wavelength setting was accidentally changed from 280nm to 300nm before the student made the measurement.

O

AP Chemistry




The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050 M Co2+

(aq) ?

A There was distilled water in the cuvette when the student put the standard solution in it. B There were a few drops of the 0.100 M Co2+

(aq) standard solution in the cuvette when the student put the 0.050 M standard solution in it.

C The student used a cuvette with a longer path length than the cuvette used for the other standard solutions.

D The student did not run a blank between the 0.050 M Co2+(aq) solution and the one

before it.

O

AP Chemistry


3. Fe3+(aq) + KSCN(s) → FeSCN2+

(aq) + K+(aq)

To determine the moles of Fe3+(aq) in a 100. mL sample of an unknown solution, excess

KSCN(s) is added to convert all the Fe3+(aq) into the dark red species FeSCN2+(aq), as

represented by the equation above.

The absorbance of FeSCN2+(aq) at different concentrations is shown in the graph below.

If the absorbance of the mixture is 0.20 at 453 nm, how many moles of Fe3+(aq) were present

in the 100. mL sample? (Assume that any volume change due to adding the KSCN(s) is negligible.)

A 4 x 10-4 mol B 3 x 10-4 mol C 4 x 10-6 mol D 3 x 10-6 mol

4. Using a spectrophotometer, a student measures the absorbance of four solutions of CuSO4 at a given wavelength.

The collected data is given in the table opposite.

Which of the following is the most likely explanation for the discrepant data in trial 4 ?

A The solution was at a lower temperature than the solutions in the other trials. B The measurement was made using a different spectrophotometer that uses a cell with a longer path length.

C The solution was saturated and the flow of light through the solution was restricted.

D The concentration of the solution was actually lower than 0.150M .

O

O

AP Chemistry


5. A student prepared five solutions of CuSO4 with different concentrations, and then filled five cuvettes, each containing one of the solutions.

The cuvettes were placed in a spectrophotometer set to the appropriate wavelength for maximum absorbance. The absorbance of each solution was measured and recorded.

The student plotted absorbance versus concentration, as shown in the figure above.

Which of the following is the most likely explanation for the variance of the data point for the 0.600 M CuSO4 solution?

A The cuvette into which the 0.600 M solution was placed had some water droplets inside. B The cuvette into which the 0.600 M solution was placed was filled slightly more than the other cuvettes.

C The wavelength setting was accidentally moved away from that of maximum absorbance.

D The cuvette used for the 0.600 M solution had not been wiped clean before being put in the spectrophotometer.

O

AP Chemistry


6. Cu(s) + 4 HNO3(aq) → Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)

Each student in a class placed a 2.00 g sample of a mixture of Cu and Al in a beaker and placed the beaker in a fume hood.

The students slowly poured 15.0 mL of 15.8 M HNO3(aq) into their beakers. The reaction between the copper in the mixture and the HNO3(aq) is represented by the equation above.

The students observed that a brown gas was released from the beakers and that the solutions turned blue, indicating the formation of Cu2+

(aq). The solutions were then diluted with distilled water to known volumes

To determine the number of moles of Cu in the sample of the mixture, the students measured the absorbance of known concentrations of Cu(NO3)2(aq) using a spectrophotometer.

A cuvette filled with some of the solution produced from the sample of the mixture was also tested. The data recorded by one student are shown in the table opposite.

On the basis of the data provided, which of the following is a possible error that the student made?

A The Cu(NO3)2(aq) from the sample of the mixture was not diluted properly. B The spectrophotometer was calibrated with tap water instead of distilled water.

C The student labeled the cuvettes incorrectly, reversing the labels on two of the solutions of known concentration.

D The spectrophotometer was originally set to an inappropriate wavelength, causing the absorbance to vary unpredictably

O

AP Chemistry


3.9 & 3.11 - 3.13 FRQ1. A student investigates the reaction between Ag(s) and HNO3(aq) represented by the equation below. 3 Ag(s) + 4 HNO3(aq) → 3 AgNO3(aq) + NO(g) + 2 H2O(l)

The student runs the reaction using a 3 to 4 mole ratio of Ag(s) to HNO3(aq) . Suggest a method the student can use to isolate solid AgNO3 from the other products of the reaction.

The student can evaporate the water, leaving behind solid silver nitrate.

2. To spectrophotometrically determine the mass percent of cobalt in an ore containing cobalt and some inert materials, solutions with known [Co2+] are prepared and the absorbance of each of the solutions is measured at the wavelength of optimum absorbance.

The data are used to create a calibration plot, shown opposite.

A 0.630 g sample of the ore is completely dissolved in concentrated HNO3(aq).

The mixture is diluted with water to a final volume of 50.00 mL. Assume that all the cobalt in the ore sample is converted to Co2+

(aq) .

a) What is the [Co2+] in the solution if the absorbance of a sample of the solution is 0.74?

An absorbance of 0.74 corresponds to a concentration of 0.0130 M (0.0125 M to 0.0135 M).

b) Calculate the number of moles of Co2+(aq) in the 50.00 mL solution.

1 point is earned for the correct calculation of the number of moles.

0.05000 L x (0.0130 mol Co2+/ 1.00 L) = 6.50 x 10-4 mol Co2+

c) Calculate the mass percent of Co in the 0.630 g sample of the ore.

1 point is earned for a correct calculation of the number of grams of Co that is consistent with the number of moles of Co2+ in part b). (may be implicit) 1 point is earned for a correct calculation of the mass percent that is consistent with the number of grams of Co.

6.50 x 10-4 mol x 58.93 g Co / 1.00 mol = 0.0383g Co 0.0383g Co / 0.630g ore x 100 % = 6.08 %

AP Chemistry


3. A student has 100. mL of 0.400 M CuSO4(aq) and is asked to make 100. mL of 0.150 M CuSO4(aq) for a spectrophotometry experiment.

The following laboratory equipment is available for preparing the solution: centigram balance, weighing paper, funnel, 10 mL beaker, 150 mL beaker, 50 mL graduated cylinder, 100 mL volumetric flask, 50 mL buret, and distilled water.

a) Calculate the volume of 0.400 M CuSO4(aq) required for the preparation.

1 point is earned for the correct volume. M1V1 = M2V2

V2 = (0.150M)(0.100L) / 0.400M V2 = 0.0375 L x (1000 mL/1L) = 37.5 mL

b) Briefly describe the essential steps to most accurately prepare the 0.150 M CuSO4(aq) from the 0.400 M CuSO4(aq) using the equipment listed above.

1 point is earned for using the buret to measure 37.5 mL of 0.400 M CuSO4 solution.

1 point is earned for adding the CuSO4 solution to the volumetric flask and filling to the mark with distilled water.

The student plans to conduct a spectrophotometric analysis to determine the concentration of Cu2+

(aq) in a solution. The solution has a small amount of Co(NO3)2(aq) present as a contaminant.

The student is given the diagram below, which shows the absorbance curves for aqueous solutions of Co2+

(aq) and Cu2+(aq).

c) The spectrophotometer available to the student has a wavelength range of 400 nm to 700 nm. What wavelength should the student use to minimize the interference from the presence of the Co2+

(aq) ions?

1 point is earned for a correct wavelength. 700 nm (Any wavelength from 650 to 700 nm is acceptable.)

AP Chemistry


4. A student is given a sample of CuSO4(s) that contains a solid impurity that is soluble and colorless. The student wants to determine the amount of CuSO4 in the sample and decides to use a spectrophotometer.

First, the student prepares a calibration graph by measuring the absorbances of CuSO4(aq) solutions of known concentrations. The graph is shown below

a) The student dissolves the entire impure sample of CuSO4(s) in enough distilled water to make 100.mL of solution. Then the student measures the absorbance of the solution and observes that it is 0.30. Determine the concentration of CuSO4(aq) in the solution.

The response indicates that the absorbance is 0.15M . (Unit not required. Any value from 0.14M to 0.16M is acceptable.)

b) Calculate the number of moles of CuSO4 that were in the impure sample of CuSO4(s).

The response gives the following calculation (or an equivalent): Unit not required.

0.15 mol /1000 mL x 100. moL = 0.015 mol

c) In addition to the number of moles of CuSO4 calculated in part b) , what other quantity must be measured in order to calculate the mass percentage of CuSO4 in the impure sample of CuSO4(s) ?

The response indicates that the mass of the sample must be measured.

AP Chemistry


5. The complete photoelectron spectrum of an element in its ground state is represented below.

a) Based on the spectrum, i) write the ground-state electron configuration of the element, and

The response gives one of the following two answers.

1s2 2s2 2p6 3s2 3p6 4s2 or [Ar] 4s2

ii) identify the element.

The response indicates Ca

b) Calculate the wavelength, in meters, of electromagnetic radiation needed to remove an electron from the valence shell of an atom of the element.


The response indicates that the energy required is 0.980 x 10-18 J . The response shows a calculation similar to the following.

E = h𝛎 = h c /λ so λ = hc / E

λ = (6.626 x 10-34 J)(2.998 x 108 ms-1) / (0.980 x 10-18 J)

λ = 2.03 x 10-7 m

AP Chemistry


6. A student is given a solution of phenol red of unknown concentration. Solutions of phenol red are bright pink under basic conditions. The student analyzes a sample of the solution using a spectrophotometer set at a wavelength of 559 nm, the wavelength of maximum absorbance for phenol red.

The measured absorbance of the phenol red solution at 559 nm is 0.35.

a) Based on the calibration curve shown below, what is the concentration of the solution in micromoles per liter (μM) ?

The response gives a value between 5.2 μM and 5.6 μM.

b) If the student mixed 10mL of distilled water with 10mL of the sample, would this diluted solution have an absorbance greater than, less than, or equal to the absorbance of the original solution? Justify your answer.


The response indicates that the diluted solution would have an absorbance less than that of the original solution. The response indicates that the diluted solution would have a lower concentration and therefore a lower absorbance, in accordance with Beer’s law.

c) If the student measured the absorbance of the solution at a wavelength of 650nm, would the absorbance be greater than, less than, or equal to the absorbance of the solution at 559nm? Justify your answer.


The response indicates that the absorbance at 650 nm would be less than the absorbance at 559 nm .

The response indicates that 599 nm is the wavelength of maximum absorbance so the absorbance at any other wavelength will be lower.

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