The Foundations of Chemistry

 

stances. Suppose Avogadro’s number of CH 4 molecules, rather than just one CH 4 mole-
cule, undergo this reaction. Then the equation can be written

CH 4
2O 2 88n CO 2
2H 2 O
6.02 1023 molecules 2(6.02 1023 molecules) 6.02 1023 molecules 2(6.02 1023 molecules)
1 mol 2 mol 1 mol 2 mol

This interpretation tells us that onemole of methane reacts with twomoles of oxygen to
produce onemole of carbon dioxide and twomoles of water.

A balanced chemical equation may

be interpreted in terms of molesof

reactants and products.

3-2 Calculations Based On Chemical Equations 93

EXAMPLE 3-2 Number of Moles Formed

How many moles of water could be produced by the reaction of 3.5 mol of methane with ex-
cess oxygen (i.e.,more than a sufficient amount of oxygen is present)?

Plan

The equation for the combustion of methane

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

1 mol 2 mol 1 mol 2 mol

shows that 1 mol of methane reacts with 2 mol of oxygen to produce 2 mol of water. From
this information we construct two unit factors:

and

We use the second factor in this calculation.

Solution

__? mol H 2 O3.5 mol CH 4 7.0 mol H 2 O

You should now work Exercises 10 and 14.

We know the mass of 1 mol of each of these substances, so we can also write

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)

16.0 g 64.0 g 44.0 g 36.0 g

80.0 g reactants 80.0 g products

The equation now tells us that 16.0 grams of CH 4 reacts with 64.0 grams of O 2 to form
44.0 grams of CO 2 and 36.0 grams of H 2 O. The Law of Conservation of Matter is sat-
isfied. Chemical equations describe reaction ratios,that is, the mole ratiosof reactants
and products as well as the relative massesof reactants and products.

2 mol H 2 O



1 mol CH 4

2 mol H 2 O



1 mol CH 4

1 mol CH 4



2 mol H 2 O

Problem-Solving Tip:Are you Using a Balanced Equation?

Never start a calculation involving a chemical reaction without first checking that the

equation is balanced.

Please don’t try to memorize

unit factors for chemical reactions;

rather, learn the general methodfor

constructing them from balanced

chemical equations.

A balanced equation may be

interpreted on a massbasis.