The Foundations of Chemistry

The ratio of atoms in the simplest formula must be a whole-number ratio(by definition). To con-

vert the ratio 1
 1 
1.5 to a whole-number ratio, each number in the ratio was multiplied by 2,

which gave the simplest formula Na 2 S 2 O 3.

You should now work Exercise 53.

72 CHAPTER 2: Chemical Formulas and Composition Stoichiometry

Millions of compounds are composed of carbon, hydrogen, and oxygen. Analyses for

C and H can be performed in a C-H combustion system (Figure 2-10). An accurately

Problem-Solving Tip:Know Common Fractions in Decimal Form

As Example 2-13 illustrates, sometimes we must convert a fraction to a whole number

by multiplication by the correct integer. But we must first recognize which fraction is

represented by a nonzero part of a number. The decimal equivalents of the following

fractions may be useful.

Decimal Equivalent To Convert to Integer,

(to 2 places) Fraction Multiply by

0.50 ^12  2

0.33 ^13  3

0.67 ^23  3

0.25 ^14  4

0.75 ^34  4

0.20 ^15  5

The fractions 5 ^2 , ^35 , ^45 are equal to 0.40, 0.60, and 0.80, respectively; these should be mul-

tiplied by 5.

When we use the procedure given in this section, we often obtain numbers such as

0.99 and 1.52. Because the results obtained by analysis of samples usually contain some

error (as well as roundoff errors), we would interpret 0.99 as 1.0 and 1.52 as 1.5.

Figure 2-10 A combustion train used for carbon-hydrogen analysis. The absorbent for

water is magnesium perchlorate, Mg(ClO 4 ) 2. Carbon dioxide is absorbed by finely divided

sodium hydroxide supported on glass wool. Only a few milligrams of sample is needed for

analysis.

O 2

Sample

Furnace

H 2 O absorber

CO 2 absorber