The Foundations of Chemistry

carbon dioxide molecule, CO 2 , contains one C atom and two O atoms. Percentage is the
part divided by the whole times 100 percent (or simply parts per 100), so we can repre-
sent the percent composition of carbon dioxide as follows:

2-8 Percent Composition and Formulas of Compounds 67

% C
100%
100%
100% 27.3%

% O
100%
100%
100% 72.7% O

2(16.0 amu)



44.0 amu

2 
AW of O



MW of CO 2

mass of O



mass of CO 2

12.0 amu



44.0 amu

AW of C



MW of CO 2

mass of C



mass of CO 2

One moleof CO 2 (44.0 g) contains one moleof C atoms (12.0 g) and two molesof O
atoms (32.0 g). We could therefore have used these masses in the preceding calculation.
These numbers are the same as the ones used—only the units are different. In Example
2-11 we shall base our calculation on one molerather than one molecule.

EXAMPLE 2-11 Percent Composition

Calculate the percent composition by mass of HNO 3.

Plan

We first calculate the mass of one mole as in Example 2-7. Then we express the mass of each
element as a percent of the total.

Solution

The molar mass of HNO 3 is calculated first.

Number of Mol of Atoms
Mass of One Mol of Atoms Mass Due to Element

1
H 1
1.0 g 1.0 g of H

1
N 1
14.0 g 14.0 g of N
3
O 3
16.0 g 48.0 g of O

Mass of 1 mol of HNO 3 63.0 g

Now, its percent composition is

% H
100%
100% 1.6% H

% N
100%
100% 22.2% N

% O
100%
100% 76.2% O

Total100.0%

You should now work Exercise 46.

Nitric acid is 1.6% H, 22.2% N, and 76.2% O by mass. All samples of pure HNO 3
have this composition, according to the Law of Definite Proportions.

48.0 g



63.0 g

mass of O



mass of HNO 3

14.0 g



63.0 g

mass of N



mass of HNO 3

1.0 g



63.0 g

mass of H



mass of HNO 3

As a check, we see that the percentages

add to 100%.

When chemists use the % notation,

they mean percent by mass unless they

specify otherwise.