The Foundations of Chemistry

The following examples show the relations between numbers of molecules, atoms, or
formula units and their masses.

EXAMPLE 2-8 Masses of Molecules

What is the mass in grams of 10.0 million SO 2 molecules?

Plan

One mole of SO 2 contains 6.02
1023 SO 2 molecules and has a mass of 64.1 grams.

Solution

_?_ g SO 2 10.0
 106 SO 2 molecules

 1.06
 10 ^15 g SO 2

Ten million SO 2 molecules have a mass of only 0.00000000000000106 g. Commonly used
analytical balances are capable of weighing to 0.0001 g.

You should now work Exercise 44.

EXAMPLE 2-9 Moles

How many (a) moles of O 2 , (b) O 2 molecules, and (c) O atoms are contained in 40.0 g of
oxygen gas (dioxygen) at 25°C?

Plan

We construct the needed unit factors from the following equalities: (a) the mass of one mole
of O 2 is 32.0 g (molar mass O 2 32.0 g/mol); (b) one mole of O 2 contains 6.02
1023 O 2
molecules; (c) one O 2 molecule contains two O atoms.

64.1 g SO 2



6.02
 1023 SO 2 molecules

When fewer than four significant

figures are needed in calculations,

Avogadro’s number may be rounded

off to 6.02
 1023.

2-7 Formula Weights, Molecular Weights, and Moles 65

TABLE 2-6 One Mole of Some Ionic Compounds

Formula A Sample with a
Compound Weight Mass of 1 Mol Contains

6.02
 1023 Naions or

sodium 58.4 58.4 g NaCl 1 mol of Na

ions

chloride 6.02
^10

(^23) Clions or
1 mol of Clions
6.02
1023 Ca^2 ions or
calcium 111.0 111.0 g CaCl 2
1 mol of Ca^2 ions
chloride 2(6.02
^10
(^23) ) Clions or
2 mol of Clions
2(6.02
1023 ) Al^3 ions or
aluminum 342.1 342.1 g Al 2 (SO 4 ) 3 2 mol of Al
3 ions
sulfate 3(6.02
^10
(^23) ) SO
4
2 ions or
3 mol of SO 42 ions
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