Chapter 3 Chemical Stoichiometry. 3.1: Atomic Mass/Weight The average mass of a sulfur atom is 32.06...

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Chapter 3

Chemical Stoichiometry

3.1: Atomic Mass/Weight

The average mass of a sulfur atom is 32.06 amu

The average mass of a sodium atom is 22.99 amu

3.2: Molecular Mass/Weight Formula Mass/Weight

•The formula of an Acetylsalicylic Acid (Aspirin) molecule is C9H8O4

•Acetylsalicylic Acid (Aspirin) has molecular mass of 180.159 amu

3.2: Molecular Mass/Weight Formula Mass/Weight

•Aluminum sulfate is an ionic compound with the formula Al2(SO4)3

•Aluminum sulfate has a formula mass of 342.153 amu.

3.3: Isotopes

• Isotopes are atoms of an element that differ only in the number of neutrons in the nucleus of the atom.

• Chlorine has two isotopes:

chlorine – 35 35Cl

chlorine – 37 37Cl

3.3: Isotopes

• In any random sample of chlorine about 1 in 4 atoms is chlorine – 37.

The atomic mass of chlorine is therefore about 35.5 amu.

3.3: Isotopes

• How do atoms of chlorine – 35 and chlorine – 37 differ?

3.4: Moles of Atoms and Avogadro’s Number

• What is a mole of atoms?

One mole of magnesium atoms contains

6.02 x 1023 Mg atoms.

• What is the mass of one mole of Mg atoms?

•The atomic mass of Mg is 24.305 amu.

•The molar mass of Mg is 24.305 grams.

3.5: Moles of Molecules

• What is a mole of molecules?

• One mole of CHCl3 (chloroform) contains 6.02 x 1023 molecules of CHCl3.

• What else does it contain?

• How many H atoms are in one mole of CHCl3?

6.02 x 1023 H atoms

• How many Cl atoms are in one mole of CHCl3?

3(6.02 x 1023) Cl atoms

• What is the molar mass of CHCl3?

119.377g

Key Concept

• You have one mole of ascorbic acid (Vitamin C) C6H8O6. What else do you have?

• 6.02 x 1023 molecules of C6H8O6

• 176.13 grams of C6H8O6

• 6 moles C or 6(6.02 x 1023) C atoms

• 8 moles H or 8(6.02 x 1023) H atoms

• 6 moles O or 6(6.02 x 1023) O atoms

Key Concept

• If you have a certain number of moles of any compound you can always find the moles of each element present?

• How many mol C are in 0.80 mol of CO2?

• How many mol O are in 0.80 mol of CO2?

• How many mol H are in 1.2 mol of C12H22O11?

• How many mol S are in 3.8 mol of Cr(SO4)3?

• How many mol O are in 3.8 mol of Cr(SO4)3?

3.6: Percentage Composition or Percent by Mass

•What is the percent by mass of hydrogen in water?

3.6: Percentage Composition or Percent by Mass

A glass of water contains 126g of water. How many grams of hydrogen are present?

3.7: Derivation of Formulas•To calculate a formula of a substance we often use a mole ratio.

3.7: Derivation of Formulas•To calculate a formula of a substance we often use a mole ratio.Analysis of a gas shows that it is composed of 0.090 mol carbon and 0.36 mol hydrogen. What is the empirical formula gas?

3.7: Derivation of Formulas

What is the empirical formula of a compound that is 27.29% carbon and 72.71% oxygen.

3.7: Derivation of FormulasA sample of hematite contains 34.97g of iron and 15.03g of oxygen. What is the empirical formula of hematite?

Determine the empirical and molecular formulas for a compound with the following elemental composition: 40.00% C, 6.72% H, 53.29% O. The molar mass of the compound is 180. g/mol.

C mol 3.331 C g 12.01

C mol 1 x C g 40.00

H mol 6.667 H g 1.008

H mol 1 x H g 6.72

O mol 3.331 O g 16.00

O mol 1 x O g 53.29

C 1 C 1.000 mol 3.331

C mol 3.331

H 2 H 2.001 mol 3.331

H mol 6.667

O 1 O 1.000 mol 3.331

O mol 3.331

• 12.01 + 2(1.008) + 1 (16.00) = 30.03

• 180./30.03 ≈ 6

• 6(CH2O) = C6H12O6

3.7: Derivation of Formulas

3.22g of a compound decomposes when heated into 1.96g of KCl and oxygen. What is the empirical formula of the compound?

3.8: Solutions

• Solute: The substance that dissolves (the minor component of a solution).

3.8: Solutions

• Solvent: The substance in which the solute dissolves (the major component of a solution).

KMnO4 solution

Solution: A homogeneous mixture of the solute and solvent.

3.8: Solutions

Dilute Concentrated

Concentration of a solution

Molarity(M) is the moles of solute per liter of solution.

Molarity(M) =moles of soluteLiters of solution

Preparation of Solutions

28.3 grams of nickel(II) chloride

Add water to make the desired volume (500.0 ml)

Molarity

• Calculate the molarity of an aqueous nickel (II) chloride solution containing 28.3 grams of nickel (II) chloride in 500.0 mL of solution.

Molarity The maximum solubility of lead (II)

chromate, PbCrO4, is 4.3 x 10-5 g/L. What is the molarity of a saturated solution of PbCrO4?

Molarity How many moles of sulfuric acid, H2SO4, are contained in 0.80L of a 0.050M solution of sulfuric acid?

How many grams of H2SO4 would be needed to make this solution?

Dilution• If we take a more concentrated solution we can dilute

it to a lower concentration by adding water to it.

• We determine the concentration (molarity) of the diluted solution by using the following formula.

M1V1 = M2V2

Dilution• Calculate the concentration of the resulting solution

when enough water is added to 250.0mL of 0.60M NaOH to make 300.0mL of solution?

Dilution• How much water would need to be added to 125mL of

a 1.50M solution of HCl to dilute the solution to a concentration of 0.570M?

Density

• An understanding of density is often necessary in solving various problems.

• Density is a ratio of mass to volume and therefore can be used in the conversion of mass to volume or volume to mass.

Mass Percent

• An understanding of mass percent is often necessary in solving various problems.

• Mass percent indicates the percentage of a particular substance in a mixture.

Aniline, C6H5NH2, a key ingredient in the preparation of dyes for fabrics, is produced by the reaction of C6H5Cl with a solution containing 28.2% NH3 by mass. If the density of the NH3 solution is 0.899g/cm3. What mass of NH3 is needed to prepare 125mL of

the ammonia solution?

aniline

Aniline, C6H5NH2, a key ingredient in the preparation of dyes for fabrics, is produced by the reaction of C6H5Cl with a solution containing 28.2% NH3 by mass. If the density of the NH3 solution is 0.899g/cm3. What mass of NH3 is needed to prepare 125mL of

the ammonia solution?

Stoichiometry: Calculations based on balanced equations.

• We use the balanced equation to determine a mole ratio.

• Then we use “Moles to Move”.

How many moles of Al2I6 are produced from 0.40 mol Al?

Calculate the moles of oxygen produced by the thermal decomposition of 100.0g of potassium chlorate.

• Write the equation for this reaction.

2KClO3 → 2KCl + 3O2

• The antacid milk of magnesia contains Mg(OH)2. What mass of NaOH would be needed to produce 16g of Mg(OH)2? MgCl2 + 2NaOH → Mg(OH)2 + 2NaCl

• The antacid milk of magnesia contains Mg(OH)2. What mass of NaOH would be needed to produce 16g of Mg(OH)2?

MgCl2 + 2NaOH → Mg(OH)2 + 2NaCl

What mass of oxygen is required to burn 702g of octane?

What mass of oxygen is required to burn 702g of octane?Write the balanced equation for this reaction.

What mass of oxygen is required to burn 702g of octane?2C8H18 + 25O2 → 16CO2 + 18H2O

•What volume of 0.750M HCl can be made from 25.0g of NaCl?

NaCl + H2SO4 → HCl + NaHSO4

•What volume of 0.2089M KI solution reacts with 43.88mL of 0.3842M Cu(NO3)2?

2Cu(NO3)2 + 4KI → 2CuI + I2 + 4KNO3

Percentage Yield

Percent Yield = actual yieldtheoretical yield

Percentage yield can be a part of stoichiometry problems

Percentage Yield

• 1.274g of CuSO4 produces 0.392g of Cu. What is the percentage yield?

CuSO4 + Zn → Cu + ZnSO4

Percentage Yield

• 1.274g of CuSO4 produces 0.392g of Cu. What is the percentage yield?

CuSO4 + Zn → Cu + ZnSO4

Limiting Reagents

• A limiting reagent (reactant) is the reactant that is used up in a chemical reaction.

• All other reactants are said to be in excess.

Limiting Reagent

• 195g NaHCO3 and 152ml 3.0M HC2H3O2

NaHCO3 + HC2H3O2 → NaC2H3O2 + H2CO3

Limiting Reagent

NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2

Limiting Reagent

• Which reactant is limiting?

Limiting Reagent

• How many grams of CO2 are produced?

Limiting Reagent

• How many grams of CO2 are produced?

• How many grams of NaHCO3 remain after the reaction?

Limiting Reagent

• How many grams of NaHCO3 remain after the reaction?

A mixture of 5.0g H2 and 10.0g of O2 is ignited forming water. How much water will the reaction produce?

Titration• Titration is a common laboratory method of quantitative

chemical analysis that is used to determine the unknown concentration of a reactant.

• Because volume measurements play a key role in titration, it is also known as volumetric analysis.

• A reagent, called the titrant, is placed in a calibrated buret and reacted with another solution.

• One of the solutions is of known concentration (a standard solution). It is used to react with a solution whose concentration is not known.

• An indicator is used to identify the endpoint of the reaction. The point at which the reaction is complete. The endpoint is often indicated by a permanent change in color due to the indicator.

Titration

Before endpoint

Endpoint

Overshoot

Titration (Acid – Base)

0.5105g NaOH is dissolved in water and titrated to an endpoint with 48.47 mL of HCl. What is the concentration of HCl?

NaOH + HCl → NaCl + H2O

A solution of 20.00mL of oxalic acid was titrated to an endpoint with 23.23mL of 0.09113M potassium permanganate. What is the

concentration of the oxalic acid solution.

5H2C2O4 + 2KMnO4 + 3H2SO4 → 2MnSO4 + K2SO4 + 10CO2 + 8H2O

How many grams of a sample containing 75.0% calcium hydroxide by mass is required to react with the acetic acid in 25.0mL of a solution having a density of 1.065 g/mL and

containing 58.0% acetic acid by mass?

Ca(OH)2 + 2HC2H3O2 → Ca(C2H3O2)2 + 2H2O

Chapter 3 Chemical Stoichiometry. 3.1: Atomic Mass/Weight The average mass of a sulfur atom is 32.06...

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