The Foundations of Chemistry

The amount of substance that contains the mass in grams numerically equal to its

formula weight in amu contains 6.022
 1023 formula units, or one moleof the sub-

stance. This is sometimes called the molar massof the substance. Molar mass is

numerically equalto the formula weight of the substance (the atomic weight for atoms

of elements) and has the units grams/mole.

One mole of sodium hydroxide is 40.0 g of NaOH, and one mole of acetic acid is 60.0 g

of CH 3 COOH. One mole of any molecular substance contains 6.02
 1023 molecules of

the substance, as Table 2-5 illustrates.

Because no simple NaCl molecules exist at ordinary temperatures, it is inappropriate

to refer to the “molecular weight” of NaCl or any ionic compound. One mole of an ionic

compound contains 6.02
 1023 formula unitsof the substance. Recall that one formula

unit of sodium chloride consists of one sodium ion, Na, and one chloride ion, Cl. One

mole, or 58.4 g, of NaCl contains 6.02
 1023 Naions and 6.02
 1023 Clions (Table

2-6).

The mole concept, together with Avogadro’s number, provides important connections

among the extensive properties mass of substance, number of moles of substance, and

number of molecules or ions. These are summarized as follows.

64 CHAPTER 2: Chemical Formulas and Composition Stoichiometry



(mol

F

ar

W

m

A

ass A)



m

g

ol

A

A

 

Av

n

o

u

g

m

a

b

d

e

ro

r

’s



Formula units of A



mol A

Moles A

Moles A

Mass A

Formula units of A

h

(^66)
(^66)
(^66)
(^66)
6
g
h
(^66)
(^66)
(^66)
(^66)
6
g
TABLE 2-5 One Mole of Some Common Molecular Substances
Molecular A Sample with a
Substance Weight Mass of Contains
6.02
1023 H 2 molecules or
hydrogen 2.0 2.0 g H (^2) (contains 21 mol of H
^2 6.02molecules
1023 H
atoms or 2 mol of H atoms)
6.02
1023 O 2 molecules or
oxygen 32.0 32.0 g O 2 1 mol of O^2 molecules
(contains 2
6.02
1023 O
atoms or 2 mol of O atoms)
6.02
1023 CH 4 molecules or
methane 16.0 16.0 g CH (^4) (contains 6.021 mol of CH
^4 molecules 1023 C atoms
and 4
6.02
1023 H atoms)
6.02
1023 CH 3 COOH
acetic acid 60.0 60.0 g CH 3 COOH molecules or 1 mol of
(vinegar) CH 3 COOH molecules
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