# Partial Pressure and Mole Fraction

Let's consider a container which has different gases in it. According to the Dalton's law of partial pressures, the partial pressures of the gases present in a gaseous mixture can be added to get the net total pressure of the mixture. But, what is partial pressure? Partial pressure is the pressure exerted by a particular gas in a mixture of gases. Now that you know what partial pressure is, take a look at the boxed information which summarizes the whole idea of Dalton's law of partial pressures.

# Dalton's Law of Partial Pressures

Ptotal= PA + PB + PC + PD + . . . (at constant volume & temperature)

Ptotalâ€“Total pressure

PA , PB , PC , PD , . . . represent the partial pressures of gases A, B, C, D, and so on.

Another concept you have to understand is mole fraction. The mole fraction of a gas is the fraction or ratio of moles of that particular gas against the total number of moles of gases present in the mixture. Mole fraction is defined as follows:

Example 6-3

A 1 liter flask contains 0.4 mol of helium and 1.2 moles of hydrogen gas. Find the mole fractions and partial pressures of both gases, if the total pressure of the mixture is 790 mmHg.

Solution

The total number of moles of gases present in the container is 0.4 + 1.2 = 1.6 moles

The mole fraction of helium = 0.4/1.6 = 0.25

The mole fraction of hydrogen = 1.2/1.6 = 0.75

Notice that the sum of the mole fractions is always one. If it is not, you probably made an error somewhere in your calculation. Here,

0.25 + 0.75 = 1.0

Next, we have to find the partial pressures of the gases. We know that the partial pressures of the gases should add up to get the total pressure of the gases. Now that we know the total pressure and the mole fractions, we can calculate the partial pressures of helium and hydrogen.

Partial pressure of gas A = mole fraction of gas A x total pressure

Partial pressure of helium = 0.25 x 790 mmHg = 197.5 mmHg

Partial pressure of hydrogen = 0.75 x 790 mmHg = 592.5 mmHg