Page 153 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 10

Stoichiometry

10.1. MOLE-TO-MOLE CALCULATIONS
In chemical work, it is important to be able to calculate how much raw material is needed to prepare a certain
quantity of products. It is also useful to know if a certain reaction method can prepare more product from a given
quantity of material than another reaction method. Analyzing material means finding out how much of each
element is present. To do the measurements, often we convert parts of the material to compounds that are easy to
separate, and then we measure those compounds. All these measurements involve chemical stoichiometry, the
science of measuring how much of one thing can be produced from certain amounts of others.
From the practical viewpoint of a student, this chapter is extremely important. The calculations introduced
here are also used in the chapters on gas laws, solution chemistry, equilibrium, and other topics.
In Chap. 8, the balanced chemical equation was introduced. The equation expresses the ratios of numbers
of formula units of each chemical involved in the reaction. Thus, for the reaction of aluminum with oxygen to
produce aluminum oxide

4Al + 3O 2 −→ 2Al 2 O 3

the equation states that the chemicals react in the ratio of four atoms of aluminum with three molecules of oxygen
(O 2 ) to produce two formula units of aluminum oxide. Thus, if eight atoms of aluminum react, they will react
with six molecules of oxygen, and four formula units of aluminum will be produced.
The balanced chemical equation may also be used to express the ratios of moles of reactants and products
involved. Thus, for the reaction whose equation is given above, 4 mol of Al reacts with 3 mol of O 2 to produce
2 mol of Al 2 O 3 . It is also true that 8 mol of aluminum can react with 6 mol of oxygen to produce 4 mol of Al 2 O 3 ,
and so on.

EXAMPLE 10.1. How many moles of Al 2 O 3 can be prepared from the reaction of 0.450 mol of O 2 plus sufficient Al?

Ans. The first step in any stoichiometry problem is to write the balanced chemical equation:
4Al + 3O 2 −→ 2Al 2 O 3
Then the coefficients in the balanced chemical equation can be used as factors in the factor-label method to convert
from moles of one chemical to moles of any other in the equation:

2 mol Al 2 O 3
0.450 mol O 2 = 0.300 mol Al 2 O 3
3 mol O 2
In the problem given before Example 10.1 with 4 mol of aluminum, it was not necessary to use the factor-label
method; the numbers were easy enough to work with. However, when the numbers get even slightly complicated,
it is useful to use the factor-label method. Note that any of the following factors could be used for this equation,
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