Chemistry, Third edition

160 10 · GASES, LIQUIDS AND SOLIDS

Fig. 10.8Avogadro’s law.

Dalton’s law of partial pressures

This law applies to gases which do not react chemically. It states that

The total pressure of a gas mixture is the sum of the partial pressures of the

individual gases in the mixture.

Thepartial pressureof a gas in a mixture is defined as the pressure that gas

would exert if it alone occupied the container.

Mathematically, Dalton’s law may be expressed as

PTpApBpC...

wherePTis the total pressure of the mixture and pA,pB,pC... are the partial

pressures of the gases A, B, C... in the mixture.

An example will show how easy it is to use Dalton’s law. Suppose we add nitrogen

to an initially empty gas cell so that the pressure of the nitrogen in the cell becomes

1.0 atm. If we now add an amount of oxygen (such that the pressure of oxygen if it

alone occupied the gas cell would be 0.5 atm), a measurement would show that the

total pressure of the mixture is 1.5 atm. This is predicted by Dalton’s law:

PTpN 2 pO 2 1.00.51.5 atm

Dalton’s law ‘works’ because the gaps between gas molecules are so large that the

molecules of the constituent gases do not interfere with each other. The total pres-

sure of the mixture is then the sum of the pressures exerted by the individual gases.

The most important gas mixture is, of course, the air around us (Table 10.1).

Table 10.1Composition of dry air at sea level

Gas Average percentage Average percentage

(by volume) (by mass)

Nitrogen N 2 78.05 75.50

Oxygen O 2 21.00 23.21

Argon Ar 0.94 1.28

Carbon dioxide CO 2 0.03 0.04

Neon Ne 0.0015 0.001

Avogadro’s law

0.50 g of zinc reacts completely with excess dilute sulfuric acid at 20 °C. Calculate the

volume of hydrogen produced at this temperature. (The molar volume of a gas at 20 °C and

1 atm pressure is 24 dm^3 ;m(Zn)65 u.)

Exercise 10G