The Foundations of Chemistry

Plan

The balanced equation tells us that one mole of CH 4 produces one mole of CO 2.

CH 4 
 2O 2 88nCO 2 
 2H 2 O

1 mol 2 mol 1 mol 2 mol

16.0 g 2(32.0 g) 44.0 g 2(18.0 g)

Solution

__?g CO 2 6.00 mol CH 4 2.64 102 g CO 2

You should now work Exercise 20.

Reaction stoichiometry usually involves interpreting a balanced chemical equation to

relate a givenbit of information to the desiredbit of information.

EXAMPLE 3-7 Mass of a Reactant Required

Phosphorus, P 4 , burns with excess oxygen to form tetraphosphorus decoxide, P 4 O 10. In this

reaction, what mass of P 4 reacts with 1.50 moles of O 2?

Plan

The balanced equation tells us that one mole of P 4 reacts with five moles of O 2.

P 4 
5O 2 88nP 4 O 10

1 mol 5 mol 1 mol

mol O 2 88n mol P 4 88n mass P 4

Solution

__?g P 4 1.50 mol O 2 37.2 g P 4

You should now work Exercise 24.

The possibilities for this kind of problem solving are limitless. Before you continue, you

should work Exercises 8–25 at the end of the chapter.

THE LIMITING REACTANT CONCEPT

In the problems we have worked thus far, the presence of an excess of one reactant was

stated or implied. The calculations were based on the substance that was used up first,

called the limiting reactant.Before we study the concept of the limiting reactant in stoi-

chiometry, let’s develop the basic idea by considering a simple but analogous nonchemical

example.

Suppose you have four slices of ham and six slices of bread and you wish to make as

many ham sandwiches as possible using only one slice of ham and two slices of bread per

sandwich. Obviously, you can make only three sandwiches, at which point you run out of

3-3

124.0 g P 4



mol P 4

1 mol P 4



5 mol O 2

44.0 g CO 2



1 mol CO 2

1 mol CO 2



1 mol CH 4

96 CHAPTER 3: Chemical Equations and Reaction Stoichiometry

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 5.5, Limiting Reactants.