Dalton’s Law of Partial Pressure

Dalton’s Law of Partial Pressure says that the total pressure of a mixture of gases equals the sum of the pressures that each would exert if it was present alone. The equation for Dalton’s Law of Partial Pressure is: the total pressure of a mixture of gases is equal to the pressure of gas A if it were present alone, referred to as the partial pressure of gas A, plus the pressure of gas B if it were present alone, plus the pressure of gas C, etc., etc.

daltons-law

In the following illustration, we have gas A in a container by itself exerting a pressure PA. Gas B (the yellow particles) is in a container by itself, exerting a pressure of PB, and Gas C is in a container by itself (the blue particles) exerting a pressure of PC. If we put all three of these gases in the same container, then the total pressure of that container is going to be the sum of the individual partial pressures of each gas. The total pressure, then, is going to be the pressure of gas A, plus the pressure of gas B, plus the pressure of gas C.

daltons-law-image

 

Dalton’s Law of Partial Pressure flows from Kinetic Molecular Theory. Since gas particles are assumed to not take up any volume, each gas particle has access to the entire volume of the container as if it were alone, and no other gas were present.

Let’s work through some sample problems.

Sample Problem

A mixture contains carbon dioxide with a partial pressure of 125 mm Hg, and oxygen with a partial pressure of 275 mm Hg. What is the total pressure of the mixture?

We know that the total pressure of the mixture is the sum of the individual partial pressures of the gases, according to Dalton’s Law of Partial Pressure:

400mmHg

The total pressure of the mixture is 400. mm Hg.

Sample Problem

A mixture of argon and neon has a total pressure of 1.50 atm. If the partial pressure of neon is 1.25 atm, what is the partial pressure of argon?

daltons-law

In this example, we are given the total pressure as 1.50 atm, and the partial pressure of neon, 1.25 atm. The total pressure 1.50 atm is the sum of the partial pressures of the two gases, neon 1.25 atm, and the partial pressure of argon.

ptotal

Therefore, the partial pressure of argon is equal to the total pressure minus the partial pressure of neon.

25atm

The partial pressure of argon is 025 atm.