UNIT 3 – Chemical Quantities

Chapter 5 Counting Atoms and Molecules – The Mole

How does the mass of a substance relate to the number of atoms in the substance?

Recall: Atomic mass units.

Atomic mass units are a ____________________________________________________

________________________________________________________________________

________________________________________________________________________

The mass of all other atoms are measured relative to carbon-12

ISOTOPES

Most elements are made up of 2 or more isotopes.

Because isotopes have different #s of neutrons, _________________________________.

This must be accounted for when ____________________________________________.

For example Mg has 3 naturally occurring isotopes.

___________________________________________________________________________________________________________.

These isotopes are present in different amounts.

In nature _______________________________________________________________.

The relative amount of each isotope is called the ________________________________.

To determine the mass of a Mg atom the isotopic abundance must be accounted for.

AVERAGE ATOMIC MASS

The _____________________________________________ of an element is the

_______________________________________________________________________.

The mass given for each element on the periodic table ___________________________.

To determine the _____________________________________________, chemists use a

device called a ___________________________________________________________

Once the data is obtained the ______________________________________ can be

calculated by finding the ____________________________________.

For Mg:

Isotope Isotopic Abundance Mass (u)

23.985041700

24.98583692

25.982592929

Average Atomic Mass

=

=

=

You try chlorine:

Isotope Isotopic Abundance Mass (u)

35Cl 75.78% 34.96885268

37Cl 24.22% 36.96590259

If you know the average atomic mass of an element and the masses of its isotopes you

can ____________________________________________________________________.

Example:

Boron exists as two naturally occurring isotopes: 10

B (10.01u) and 11

B (11.01u). Boron

has an average atomic mass of 10.81u. Calculate the isotopic abundance of each isotope.

The Mole

In chemistry as in other aspects of life it is sometimes more convenient to count in groups

of items rather than count items individually.

Quantity Amount

In chemistry chemical reactions are __________________________________________.

To count the # of atoms in a chemical reaction individually is inconvenient so chemists

group atoms into groups.

Chemists count in ____________________________________________

________________________________________________________________________

This value is known as the __________________________________________________

The mole is defined as:

________________________________________________________________________

________________________________________________________________________

How big is the Avogadro constant?

Try the Thought lab on pg. 175.

- Canada covers 9976140 km2

- Distance to moon is 384,403 km

- Length of $5 is 152.4 mm

- The worlds oceans hold 1.37x109

km3 water.

- One teaspoon is equal to 5ml

- Average size apple has a mass of 0.15 kg

- Mass of the Earth is 5.9742 × 1024

kg

- Current population of Earth is 6,654,181,537

Converting Moles to Number of Particles

N =

n =

NA =

Problem 1. A sample contains 2.25 moles of carbon dioxide, CO2.

a. How many molecules are in the sample?

b. How many atoms are in the sample?

Problem 2. How many moles of nitrogen dioxide are there in a sample containing 4.35 x

1024

molecules?

Molar Mass

When we measure the mass of a substance we express its mass in grams.

You have already learned that one atom of ______________ has a mass of exactly _____

and that ______________ atoms has a mass of exactly _______.

This means:

__________________________________________________________________

_________________________________________________________________.

This is known as ________________________________________________________.

Molar mass can be applied to any substance and is equivalent to the

_______________________________________________________________________.

Molar mass of Al

Molar mass of HNO3 or MHNO

Using Molar Mass

The following equation will allow you to solve problems involving

________________________________________________________________________

Problem 1.

A flask contains 0.950 mol of carbon monoxide, CO. What is the mass of CO in this

sample?

Problem 2.

How many moles of acetic acid CH3COOH are in a 15.2g sample?

Problem 3.

What is the mass of 6.35 x 1024

molecules of copper (II) nitrate.

Number

of

Particles

Moles Mass of

substance

Chapter 6 – Chemical Proportions in Compounds

In the late 1700’s Joseph Proust discovered that samples of copper (II) carbonate always

contained the same proportion of Cu, C and O.

This led to the ___________________________________________________________

– the elements in a chemical compound are always present

_______________________________________________________________________

The mass of an element in a compound, expressed as a % of the total mass is known as an

elements ________________________________.

Ex H2O

The law of definite proportions _____________________ that the elements in

compounds are always the same.

CO

CO2

If we state all of the mass percents for a compound we call it

_______________________________________________________________________

A compound with a mass of 48.72g is found to contain 32.69g of Zn and 16.03g of S.

What is the % composition?

Calculating % composition from a formula

Pyrite (aka _________________________) is a compound with the chemical formula

_______________. If you wanted to extract the iron and know how much to expect, you

first need to know the _____________________________.

Calculate the % composition for ________________.

______________________________ C9H8O is the molecule responsible for the

____________________________________________________________________.

Find the % composition of this compound.

Empirical Formula of a Compound

The _____________________________________ of a compound shows the

____________________________________________ of the elements in the compound.

Ex H2O2

The ____________________________________________ of a compound shows the

___________________________________________ of each element in the compound.

The relationship between the 2 types of formulas is:

Determining Empirical Formulas

To determine an empirical formula you need to use the concepts of

________________________________________________________________________

Ex1. Calculate the empirical formula for a compound that is 85.6% C and 14.4% H.

Ex2. The percentage composition of a fuel is 81.7% carbon and 18.3% hydrogen. Find

the empirical formula.

Molecular Formula of a Compound

Being able to identify unknown substances as many uses. Think forensic science.

Since many substances have the same empirical formulas, we need to be able to

determine the _____________________________________ for a substance. To do this

we use _________________________.

Chemists use a ___________________________________ to determine the molar mass

of a substance. Then:

Ex. 1 The empirical formula for ribose (a sugar) is CH2O. The molar mass of ribose is

150g/mol (by experiment). What is the molecular formula of ribose?

Finding Formulas by Experiment

There are many ways to determine formulas by experiment.

The Carbon-Hydrogen Combustion Analyzer

This is a device used to ____________________________________________________.

How it works

Stream of ______________________________________ containing compounds.

CH compound will react completely to produce __________________________

All the H2O is absorbed by ___________.

Final mass – initial mass = mass of H2O. All ____________________________.

Use ______________________ of H in H2O to determine mass of H

All the CO2 is absorbed by __________________. All C ends up here.

Use _____________________________ of CO2 to determine mass of C.

If original compound contained a third element, then:

Ex1. A 1.000g sample of a pure compound containing only C and H was combusted.

0.6919g of water and 3.338g of carbon dioxide were produced.

a. Calculate the masses of C and H in the sample.

b. Find the empirical formula of the compound.

Hydrated Ionic Compounds

Many ______________________ crystallize from an aqueous solution, with H2O

molecules ___________________________________________________________.

These are called ____________________________.

_______________________ have a specific # of _______________________ chemically

bonded to each formula unit.

Chemical Name Formula

Compounds that have no H2O molecules are called ______________________________

Just remember: when doing calculations you must

________________________________________________________________________

Ex 1.

A 50.0g sample of a hydrate of barium hydroxide Ba(OH)2xH2O contains 27.2g of

Ba(OH)2.

a. Calculate the percent by mass of water in Ba(OH)2xH2O

b. Find the value of x in Ba(OH)2.

Chapter 7 Quantities in Chemical Reactions

Stoichiometry

Stoichiometry is the study of the

________________________________________________________________________

It all begins with ___________________________________________, which are

essential for making calculations related to chemical reactions.

For example, the __________________________________ (used to make ammonia gas)

is based on the following balanced equation:

What does this equation tell you?

1. s

2. s

3. If we multiply each by 12 we could say: 1 dozen molecules of nitrogen gas reacts

with 3 dozen molecules of hydrogen gas to produce 2 dozen molecules of

ammonia gas.

4.

The relationship between moles in a balanced chemical equation are known as MOLE

RATIOS

We use mole ratios to solve problems.

1. From the equation above, how much ammonia gas would be produced from 3.5

mol of hydrogen gas?

2. When carbon and oxygen combine 2 reactions are possible. The product can be

CO2 or CO.

a. Write the balanced chemical equations.

b. How much oxygen is needed to react with 0.75mol carbon to produce

CO2?

c. How much oxygen is needed to react with 0.75mol carbon to produce CO?

Mole Ratios can also be used to find the mass ratios for chemical equations.

N2(g) + 3H2(g) 2NH3(g)

Balanced

Equation

# of Particles

Amount

Mass = n x M

Total mass

Solving Stoichiometric Problems

Follow this process:

1.

2.

3.

4.

Ex 1. Mass to Mass

Astronauts must be able to remove the CO2 from their spacecraft. If not hypoxia (lack of

O2) would occur. CO2 is removed using the following chemical reaction: CO2(g) +

2LiOH(s) Li2CO3(g) + H2O(g) . If an astronaut produces 1.50x103 g of CO2 a day, what

mass of LiOH would be needed per day?

Ex 2. On Feb, 20, 2008 the US Navy shot down a spy satellite in space from a war ship

in the Pacific ocean. They claimed the satellite contained hydrazine (rocket fuel), that

could harm people if the satellite fell back to Earth. When hydrazine is combined with

dinitrogen tetroxide it produces nitrogen gas and water. If 100g of hydrazine is reacted,

how much nitrogen gas would be produced?

Ex 3. During WWI, chlorine gas was used as a weapon. When chlorine gas enters the

lungs it reacts with water and forms corrosive hydrochloric acid and oxygen gas. How

many molecules of chlorine react to produce 5.0g of HCl?

The Limiting Reactant

Chemical reaction equations give the ideal

_______________________________________________________________________.

However, the reactants for a reaction in an experiment are not necessarily a

_______________________________________________________________________.

In a chemical reaction, one of the reactants may ___________________ when the

reaction is finished. These are called ___________________. The reagent that is

______________________________ or reacted is called the ______________________,

because its quantity limit the amount of products formed.

Think of when you put gas in your car:

2C8H18 + 25O2 16CO2 + 18H2O

Ideally, ____________ of oxygen react with _________________ of octane. But in

reality __________________________________________so ___________ would be the

________________________________________. When you run out of gas

____________________________

In stoichiometric problems, _________________________________________________

________________________________________________________________________

________________________________________________________________________

Ex 1. Glucose reacts with oxygen gas to produce carbon dioxide and water during

cellular respiration. If 1.25 g of glucose reacts with 7.51g of oxygen gas, find the

limiting reactant.

Ex 2. A 2.00g piece of phosphorus is burned in a flask containing 6.91 g of oxygen gas.

What mass of tetraphosphorus decaoxide is produced?

Percentage Yield

Chemists use _____________________ to predict the ________________________ that

can be __________________ from a chemical reaction.

This amount is called the __________________________________________.

This amount is _____________________________. The ____________________

obtained in an experiment is called the ______________________________.

The ____________________ is usually less then the _______________________ and for

a variety of reasons. For example:

a.

b.

c.

d.

_________________________________________________ compares the

______________________________________________________________ (actual) to

the ____________________________________________ (theoretical).

Ex1. When 75g of nitrogen gas reacts with sufficient hydrogen gas, the theoretical yield

of ammonia is 9.10g. If 1.72 g of ammonia is obtained what is the percentage yield?

Ex 2. When Calcium carbonate is decomposed to calcium oxide and carbon dioxide there

is a 92.4% yield of calcium oxide. How many grams of calcium oxide can you expect if

12.4g of calcium carbonate is heated.

Many times substances may not be pure. The substance might contain

_______________________. If gold is 99% pure then it must contain 1% other

substances. ________________________________________ describes what proportion

_________________ of a sample is composed of a specific compound or element.

Ex 1. Your have a 13.9 g sample of “fools gold”. You heat the sample in oxygen and

produce 8.02g of iron (III) oxide. What % purity of iron pyrite was in the original

sample?

4FeS2 + 11O2 2Fe2O3 + 8SO2