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Chapter 9-Stoichiometry Stoichiometry – measures and calculates

amounts of chemicals in a reaction.

A. Mole/Mole Problems

Coefficients:

Show # of molecules in equations

Show # of moles in equations

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Example: Glucose reacts with oxygen to produce

carbon dioxide and water vapor in cellular respiration.

a. Write a balanced equation for this reaction.

b.If 0.30 moles of glucose are reacted, how many moles of carbon dioxide will be produced?

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c. If 0.15 moles of glucose are reacted, how

many moles of oxygen are reacted?

d. If 0.26 moles of water are produced, how

many moles of glucose are reacted?

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B. Mass Stoichiometry Problems

**First-Write a Balanced Equation**

1. Change to Moles

2. Use Coefficients (=Mole Ratios)

changes chemical

3. Change to what is asked for

May not need steps 1 or 3 – depends on what you are given and what is asked for.

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Examples:

1. Hydrogen peroxide (H2O2) decomposes to form water and oxygen gas. If 25.0 grams of hydrogen peroxide decompose, how many grams of oxygen will be produced?

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2. 55.0 grams of hydrogen fluoride are completely reacted with tin. How many moles of tin (II) fluoride are produced?

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C. Limiting & Excess Reactants

Reactants are not always completely used up in a reaction.

Excess Reactant = reactant that has leftovers at the end of the reaction.

Limiting Reactant = reactant that is completely used up in the reaction

**makes the least product**

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Example:

2.25 grams of PCl3 are mixed with

1.23 grams of H2O in the following reaction:

PCl3 + 3H2O H3PO3 + 3HCl

a. Calculate the limiting & excess reactants

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b. How many grams of HCl will be produced?

c. How many grams of the excess reactant

will be left?

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D. Percent Yield

Grams of Product calculated from limiting reactant = Theoretical Yield

*This is amount of product that should be produced if experiment is 100% perfect.

Amount really produced = Actual Yield

%Yield = Actual Yield x 100 Theoretical Yield

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Example:

When 14.0 grams of nitrogen gas are mixed

with 9.0 grams of hydrogen gas,

16.1 grams of ammonia gas are formed.

1. Write a balanced equation for this reaction.

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2. Calculate to find the limiting & excess

reactants.

3. What is the theoretical yield in grams of

ammonia?

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4. What is the percent yield of ammonia in this reaction?

5. How many grams of the excess reactant

are left?