Chapter 8

Reaction Stoichiometry

Deals with the mass relationships that exist

between reactants and product

In this type of chemistry, a quantity is given, and based on the chemical formula, a calculation of an unknown is completed.

Problem types fall into four different categories

Reaction Stoichiometry

A given amount in moles and an

unknown amount in mole

Mole known → mole unknown

Reaction Stoichiometry

A given amount in moles and an

unknown amount in grams

Mole known → mole unknown → gram unknown

Reaction Stoichiometry

A given amount in grams and an

unknown amount in moles

Gram known → mole known → mole unknown

Reaction Stoichiometry

A given amount in grams and an

unknown amount in grams

Gram known → mole known → mole unknown → gram unknown

Reaction Stoichiometry

The conversion factor used to relate the

number of grams of a substance to the number of moles of the substance

Always derived from the periodic table

Example: 101.96 g Al2O3 or 1Mole Al2O3 1 Mole Al2O3

101.96 g Al2O3

Molar Mass

Percent Composition

How much of a compound by mass is a

particular element?

Percent Composition

What percent by mass is hydrogen in water?

Sample Question

What is the percent composition of the

elements in Cu3(PO4)2?

Sample Question

Empirical Formula

When chemists discover new compounds in the

laboratory they need to be able to determine the chemical formula of the compound in order to replicate it.

They first discover the simplest whole number ratio of atoms in the compound This is known as the empirical formula

From this they derive the actual formula of the compound This is known as the molecular formula

Determining Chemical Formulas

This can be derived knowing either the

percent of each element in a compound or the number of grams used of each element.

Percent to Mass Mass to Mole Divide by Small Multiply till Whole

Determining the Empirical Formula

A compound consists of 25.94% N and 74.06

% O; what is its empirical formula?

Determining the Empirical Formula

A white solid consists of Na, S, and O. 32.4% is

Na, and 22.6% is S, what is the compounds empirical formula?

Determining the Empirical Formula

A scientist formed a compound out of only tin

and chlorine and it weighed 2.57 g. He knew he used 1.17 g of tin. Can you help him find the empirical formula?

Determining the Empirical Formula

Molecular Formula

Any multiple of a compounds empirical

formula may or may not be a correct molecular formula.

For example C2H4 and C3H6 both have the same ratio of Carbon to Hydrogen (3:1), however, the substances are completely different.

Molecular Formula

By dividing the molecular mass by the mass

of the empirical formula, we can obtain the multiple by which the molecular formula can be found.

Calculating Molecular Formulas

A compound has an empirical formula of CH2O

and a molar mass of 180 g / mol, what is the molecular formula of the compound?

Finding Molecular Formulas

A compound has an empirical formula of NO2

and a molar mass of 92.02 g/mol. What is its molecular formula?

Finding Molecular Formulas

A compound contains 0.70 g Cr, 0.65 g S, and

1.30 g O, the molar mass of the molecule is 392.2 g/mol. What is the molecular formula?

Practice: do 39 p 237, and 158 p. 929

Finding Molecular Formulas

End of Chapter 8

Chapter 9

A conversion factor that relates any two

substances in a chemical reaction

This information comes directly from the balanced chemical equation

Mole Ratio

Ideal = all of the reactants are converted into

products and we are not concerned about any other reactants left over

Usually the question will say one of the reactant is in excess

Ideal Stoichiometry

In the formation of water, how many moles of

O2 must react with 2 moles of H2?

Stoichiometry

How many moles of H2O will be produced by

the reaction of 10 grams of H2 reacting with excess O2?

Stoichiometry

How many moles of NH3 will be produced from

3.8 moles of N2 reacting with excess H2 according to the following equation:

N2 + 3H2 → 2NH3

Stoichiometry

N2 + 3H2 → 2NH3

Using the same equation starting with 15.9 grams of nitrogen, how many moles of H2 are required to completely react with the N2?

Stoichiometry

N2 + 3H2 → 2NH3

How many grams of NH3 will be produced if we start with 350 g of H2?

Stoichiometry

Limiting Reactants

The limiting reactant is the reactant that limits

the amount of the other reactant and the amount of product that can form in a chemical reaction.

Limiting Reactants

C + O2 → CO2

1 mole of Carbon reacts with 1 mole of O2 to yield 1 mole of CO2

or5 mole of Carbon reacts with 5 mole of O2 to yield 5 mole of CO2

But what if you started with 5 moles of Carbon and 10 moles of O2?

Limiting Reactant

C + O2 → CO2

The 5 moles of Carbon with react with the first 5 moles of O2 to yield 5 moles of CO2….but 5 moles of O2 would be left over.

If there was more Carbon to start with, more CO2 could have been formed.

Carbon in this case is a limiting reactant.

Limiting Reactant

N2H4 + 2H2O2 → N2 + 4H2O

If 0.75 moles of N2H4 is mixed with 0.50 moles of H2O2, what is the limiting reactant?

How much of the excess reactant remains unchanged?

How much of each product is formed, in moles?

Limiting Reactant

Carbon reacts with steam to produce hydrogen and carbon monoxide. a. Write a balanced chemical equation for the

reaction.

b. If 2.40 moles of Carbon are exposed to 3.10 moles of steam, identify the limiting reactant.

c. How many moles of each product are formed?

d. What mass of each product is formed?

3Fe + 4H2O → Fe3O4 + 4H2

a. When 36.0g of H2O is mixed with 167g of Fe, which is the limiting reactant?

b. What is the mass, in grams, of the Iron Oxide produced?

c. What is the mass, in grams, of the excess reactant that remains when the reaction is completed?

Percentage Yield

The theoretical yield is the maximum amount

of product that can be produced from a given amount of reactant.

The measured amount of product obtained from a reaction is called the actual yield of that product.

Percent Yield

In most reactions, the actual product is less

than the theoretical product.

This is due to side reactions or impurities in the product which must be removed.

Percent Yield

The percent yield is the ratio of the actual

yield to the theoretical yield, multiplied by 100.

Percent Yield

End of Chapter 9