The Foundations of Chemistry

Step 2: The simplest formula is CH 2 O, which has a formula weight of 30.03 amu. Because the

molecular weight of glucose is approximately 180 amu, we can determine the molecular for-

mula by dividing the molecular weight by the simplest formula weight.

n6.00

The molecular weight is six times the simplest formula weight, 6
(CH 2 O)C 6 H 12 O 6 , so

the molecular formula of glucose is C 6 H 12 O 6.

You should now work Exercises 63 and 64.

As we shall see when we discuss the composition of compounds in some detail, two

(and sometimes more) elements may form more than one compound. The Law of

Multiple Proportionssummarizes many experiments on such compounds. It is usually

stated: When two elements, A and B, form more than one compound, the ratio of the

masses of element B that combine with a given mass of element A in each of the com-

pounds can be expressed by small whole numbers. Water, H 2 O, and hydrogen peroxide,

H 2 O 2 , provide an example. The ratio of masses of oxygen that combine with a given mass

of hydrogen in H 2 O and H 2 O 2 is 1
2. Many similar examples, such as CO and CO 2 (1
 2

ratio) and SO 2 and SO 3 (2
3 ratio), are known. The Law of Multiple Proportions had

been recognized from studies of elemental composition before the time of Dalton. It pro-

vided additional support for his atomic theory.

EXAMPLE 2-17 Law of Multiple Proportions

What is the ratio of the masses of oxygen that are combined with one gram of nitrogen in the

compounds N 2 O 3 and NO?

Plan

First we calculate the mass of O that combines with one gram of N in each compound. Then

we determine the ratio of the values of 

g

g

N

O

for the two compounds.

Solution

In N 2 O 3 : 1.71 g O/g N

In NO: 1.14 g O/g N

(in N 2 O 3 )

The ratio is 

(in NO)

We see that the ratio is three mass units of O (in N 2 O 3 ) to two mass units of O (in NO).

You should now work Exercises 67 and 68.

g O



g N

3



2

1.5



1.0

1.71 g O/g N



1.14 g O/g N

g O



g N

16.0 g O



14.0 g N

_?_ g O



g N

48.0 g O



28.0 g N

_?_ g O



g N

180 amu



30.03 amu

Many sugars are rich sources in our
diet. The most familiar is ordinary
table sugar, which is sucrose,
C 12 H 22 O 11. An enzyme in our saliva
readily splits sucrose into two simple
sugars, glucose and fructose. The
simplest formula for both glucose
and fructose is C 6 H 12 O 6. They have
different structures and different
properties, however, so they are
different compounds.

76 CHAPTER 2: Chemical Formulas and Composition Stoichiometry

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