The Foundations of Chemistry

Plan

(a) The number of moles of O 2 produced can be calculated from the ideal gas equation. (b)
Then we use the balanced chemical equation to relate the known number of moles of O 2
formed and the mass of KClO 3 that decomposed to produce it.

Solution

(a)V405 mL0.405 L; P745 torr0.980 atm

(a)T25°C273°298 K

Solving the ideal gas equation for nand evaluating gives

n0.0162 mol O 2

(b) ? g KClO 3 0.0162 mol O 2 1.32 g KClO 3

The sample contained 1.32 grams of KClO 3. The percent of KClO 3 in the sample is

% KClO 3 100%100% 73.3% KClO 3

You should now work Exercise 78.

Our study of stochiometry has shown that substances react in definite mole and mass
proportions. Using previously discussed gas laws, we can show that gases also react in
simple, definite proportions by volume. For example, onevolume of hydrogen always
combines (reacts) with onevolume of chlorine to form twovolumes of hydrogen chloride,
if all volumes are measured at the same temperature and pressure

H 2 (g)  Cl 2 (g) 88n 2HCl(g)

1 volume1 volume 88n 2 volumes

Volumes may be expressed in any units as long as the same unit is used for all. Gay-Lussac
summarized several experimental observations on combining volumes of gases. The
summary is known as Gay-Lussac’s Law of Combining Volumes:

At constant temperature and pressure, the volumes of reacting gases can be expressed

as a ratio of simple whole numbers.

The ratio is obtained from the coefficients in the balanced equation for the reaction.
Clearly, the law applies only to gaseoussubstances at the same temperature and pressure.
No generalizations can be made about the volumes of solids and liquids as they undergo
chemical reactions. Consider the following examples, based on experimental observations
at constant temperature and pressure. Hundreds more could be cited.

1.32 g



1.80 g

g KClO 3



g sample

122.6 g KClO 3



1 mol KClO 3

2 mol KClO 3



3 mol O 2

(0.980 atm)(0.405 L)




0.0821 m

L

o

a

l

t



m

K

(298 K)

PV



RT

1 atm



760 torr

12-12 Mass-Volume Relationships in Reactions Involving Gases 463

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 12.7, Gas Laws and Chemical

Reactions.