The Foundations of Chemistry

Solution

One mole of O 2 contains 6.02
 1023 O 2 molecules, and its mass is 32.0 g.

(a) _?_ mol O 2 40.0 g O 2 
1.25 mol O 2

(b) _?_ O 2 molecules40.0 g O 2 

 7.52
 1023 molecules

Or,we can use the number of moles of O 2 calculated in part (a) to find the number of O 2 mol-

ecules.

_?_ O 2 molecules1.25 mol O 2 
7.52
 1023 O 2 molecules

(c) _?_ O atoms40.0 g O 2 

 1.50
 1024 O atoms

You should now work Exercise 30.

EXAMPLE 2-10 Numbers of Atoms

Calculate the number of hydrogen atoms in 39.6 g of ammonium sulfate, (NH 4 ) 2 SO 4.

Plan

One mole of (NH 4 ) 2 SO 4 is 6.02
 1023 formula units and has a mass of 132.1 g.

88n88n88n H atoms

Solution

formula units of



(NH 4 ) 2 SO 4

mol of



(NH 4 ) 2 SO 4

g of



(NH 4 ) 2 SO 4

2 O atoms



1 O 2 molecule

6.02
 1023 O 2 molecules



32.0 g O 2

6.02
 1023 O 2 molecules



1 mol O 2

6.02
 1023 O 2 molecules



32.0 g O 2

1 mol O 2



32.0 g O 2

In Example 2-10, we relate (a) grams
to moles, (b) moles to formula units,
and (c) formula units to H atoms.

66 CHAPTER 2: Chemical Formulas and Composition Stoichiometry

You should now work Exercise 38.

PERCENT COMPOSITION AND FORMULAS

OF COMPOUNDS

If the formula of a compound is known, its chemical composition can be expressed as the

mass percent of each element in the compound (percent composition). For example, one

2-8

_?_ H atoms39.6 g (NH 4 ) 2 SO 4 

 1.44
 1024 H atoms

8 H atoms



1 formula units (NH 4 ) 2 SO 4

6.02
 1023 formula units (NH 4 ) 2 SO 4



1 mol (NH 4 ) 2 SO 4

1 mol (NH 4 ) 2 SO 4



132.1 g (NH 4 ) 2 SO 4