The Foundations of Chemistry

A balanced chemical equation may be
interpreted on a molecularbasis.

92 CHAPTER 3: Chemical Equations and Reaction Stoichiometry

CALCULATIONS BASED ON CHEMICAL EQUATIONS

We are now ready to use chemical equations to calculate the relative amountsof substances

involved in chemical reactions. Let us again consider the combustion of methane in ex-

cess oxygen. The balanced chemical equation for that reaction is

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

On a quantitative basis, at the molecular level, the equation says

CH 4 
 2O 2 88n CO 2 
 2H 2 O

1 molecule 2 molecules 1 molecule 2 molecules

of methane of oxygen of carbon dioxide of water

EXAMPLE 3-1 Number of Molecules

How many O 2 molecules react with 47 CH 4 molecules according to the preceding equation?

Plan

The balancedequation tells us that oneCH 4 molecule reacts with twoO 2 molecules. We can

construct two unit factors from this fact:

and

These expressions are unit factors for thisreaction because the numerator and denom-

inator are chemically equivalent.In other words, the numerator and the denominator

represent the same amount of reaction. To convert CH 4 molecules to O 2 molecules, we

multiply by the second of the two factors.

Solution

__?O 2 molecules47 CH 4 molecules94 O 2 molecules

You should now work Exercise 8.

A chemical equation also indicates the relative amounts of each reactant and product

in a given chemical reaction. We showed earlier that formulas can represent moles of sub-

2 O 2 molecules



1 CH 4 molecule

2 O 2 molecules



1 CH 4 molecule

1 CH 4 molecule



2 O 2 molecules

3-2

Problem-Solving Tip:Balancing Chemical Equations

There is no one best place to start when balancing a chemical equation, but the follow-

ing suggestions might be helpful:

(1)Look for elements that appear in only one place on each side of the equation (in

only one reactant and in only one product), and balance those elements first.

(2)If free, uncombined elements appear on either side, balance them last.

Notice how these suggestions worked in the procedures illustrated in this section. Above

all, remember that we should neverchange subscripts in formulas, because doing so would

describe different substances. We only adjust the coefficients to balance the equation.

See the Saunders Interactive
General Chemistry CD-ROM,
Screens 5.1–5.3, Stoichiometry.

We usually cannot work with
individual molecules; a mole of a
substance is an amount we might
use in a laboratory experiment.