Dalton's Law of Partial Pressures is used to determine how much of one gas is present in a mixture of gases.

Example 1

The pressure of a mixture of nitrogen, carbon dioxide, and oxygen is 150 kPa. What is the partial pressure of oxygen if the partial pressures of the nitrogen and carbon dioxide are 100 kPA and 24 kPa, respectively?

P = P_nitrogen + P_{carbon dioxide} + P_oxygen

150 kPa = 100 kPa + 24 kPa + P_oxygen

P_oxygen = 150 kPa – 100 kPa – 24 kPa

P_oxygen = 26 kPa

Example 2

Scuba divers use a mixture of gases in their air tanks comprising approximately 80% nitrogen and 20% oxygen. If the total pressure in the air tank is 760mmHg, what is the partial pressure due to each of the gases?

If we then look at this mixture as it relates to Dalton's Law, we know that 80% of the pressure of the gas is due to the nitrogen in the mixture and 20% of the pressure is due to the oxygen in the mixture. We refer to these as partial pressures. This means at the surface, the pressure exerted on us by the nitrogen in the air mixture is 80% of 760mmHg, or 608mmHg. The pressure from the oxygen is 20% of 760mmHg, or 152mmHg. Together, these account for the 760mmHg of pressure (608mmHg N₂ + 152mmHg O₂).

Watch the following video which will help you visualize what Dalton’s Law is all about.