The Foundations of Chemistry

of the sample is 100%98.2%1.8% impurities; this gives the unit factor 

1.

1

8

00

g

g

im

s

p

am

ur

p

it

l

i

e

es

.

Solution

_?_ g NaOH45.2 g sample
44.4 g NaOH

_?_ g impurities45.2 g sample
0.81 g impurities

You should now work Exercises 82 and 83.

1.8 g impurities



100 g sample

98.2 g NaOH



100 g sample

2-12 Purity of Samples 81

Problem-Solving Tip:Utility of the Unit Factor Method

Observe the beauty of the unit factor approach to problem solving! Such questions as

“do we multiply by 0.982 or divide by 0.982?” never arise. The units always point to-

ward the correct answer because we use unit factors constructed so that units alwayscan-

cel out until we arrive at the desired unit.

Mass mixture

Vol mixture

MIXTURE

CONTAINING A

Vol A

PURE A

Mass A

Moles A

F units A

Density

mixture

Mass mixture

Vol mixture

% A

(by mass)

Parts A (by mass)

100 parts mixture (by mass)

Density A Mass A

Vol A

FW A

g A

mol A

Avogadro’s

number

F units A

 mol A







Many important relationships have been introduced in this chapter. Some of the most
important transformations you have seen in Chapters 1 and 2 are summarized in Figure
2-12.

Figure 2-12 Some important

relationships from Chapters 1

and 2. The relationships that

provide unit factors are enclosed

in green boxes.

A label from a bottle of sodium hy-

droxide, NaOH.