The Foundations of Chemistry

Using the partial and total pressures calculated in Example 12-15,

XCH 4 0.222

XH 2 0.334

XN 2 0.445

You should now work Exercise 60.

EXAMPLE 12-17 Partial Pressure, Mole Fraction

The mole fraction of oxygen in the atmosphere is 0.2094. Calculate the partial pressure of O 2
in air when the atmospheric pressure is 760. torr.

Plan

The partial pressure of each gas in a mixture is equal to its mole fraction in the mixture times
the total pressure of the mixture.

Solution

PO 2 XO 2 Ptotal

0.2094760. torr 159 torr

Dalton’s Law can be used in combination with other gas laws, as the following example
shows.

EXAMPLE 12-18 Mixture of Gases

Two tanks are connected by a closed valve. Each tank is filled with gas as shown, and both
tanks are held at the same temperature. We open the valve and allow the gases to mix.

(a) After the gases mix, what is the partial pressure of each gas, and what is the total pressure?

(b) What is the mole fraction of each gas in the mixture?

0.979 atm



2.20 atm

PN 2



Ptotal

0.734 atm



2.20 atm

PH 2



Ptotal

0.489



2.20 atm

PCH 4



Ptotal

The difference between the two

calculated results is due to rounding.

12-11 Dalton’s Law of Partial Pressures 459

Plan

(a) Each gas expands to fill the available volume, 5.00 liters plus 3.00 liters or a total volume
of 8.00 liters. We can use Boyle’s Law to calculate the partial pressure that each gas would
exert after it expands to fill 8.00 L. The total pressure is equal to the sum of the partial pres-
sures of the two gases. (b) The mole fractions can be calculated from the ratio of the partial
pressure of each gas to the total pressure.

Tank A Tank B

5.00 L of O 2

24.0 atm

3.00 L of N 2

32.0 atm