11.4: Partial pressure

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Learning Objectives

Learn and apply Dalton’s Law of Partial Pressure.

To determine the contribution of each component gas to the total pressure of a mixture of gases.

The atmosphere of Venus is markedly different from that of Earth. The gases in the Venusian atmosphere are \(96.5\%\) carbon dioxide and \(3\%\) nitrogen. The atmospheric pressure on Venus is roughly 92 times that of Earth, so the amount of nitrogen on Venus contributes pressure well over \(2700 \: \text{mm} \: \ce{Hg}\). And there is no oxygen present, so humans could not breathe there. Not that anyone would want to go to Venus—the surface temperature is usually over \(460^\text{o} \text{C}\).

Dalton's Law of Partial Pressures

Gas pressure results from collisions between gas particles and the inside walls of their container. If more gas is added to a rigid container, the gas pressure increases. The identities of the two gases do not matter. John Dalton, the English chemist who proposed the atomic theory, also studied mixtures of gases. He found that each gas in a mixture exerts a pressure independently of every other gas in the mixture. For example, our atmosphere is composed of about \(78\%\) nitrogen and \(21\%\) oxygen, with smaller amounts of several other gases making up the rest. Since nitrogen makes up \(78\%\) of the gas particles in a given sample of air, it exerts \(78\%\) of the pressure. If the overall atmospheric pressure is \(1.00 \: \text{atm}\), then the pressure of just the nitrogen in the air is \(0.78 \: \text{atm}\). The pressure of the oxygen in the air is \(0.21 \: \text{atm}\).

The partial pressure of a gas is the contribution that gas makes to the total pressure when the gas is part of a mixture. The partial pressure of nitrogen is represented by \(P_{N_2}\). Dalton's law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of all of the partial pressures of the component gases. Dalton's law can be expressed with the following equation:

\[P_\text{total} = P_1 + P_2 + P_3 + \cdots\]

The figure below shows two gases that are in separate, equal-sized containers at the same temperature and pressure. Each exerts a different pressure, \(P_1\) and \(P_2\), reflective of the number of particles in the container. On the right, the two gases are combined into the same container, with no volume change. The total pressure of the gas mixture is equal to the sum of the individual pressures. If \(P_1 = 300 \: \text{mm} \: \ce{Hg}\) and \(P_2 = 500 \: \text{mm} \: \ce{Hg}\), then \(P_\text{total} = 800 \: \text{mm} \: \ce{Hg}\).

CK12 Screenshot 14-12-1.png — Figure \(\PageIndex{1}\): Dalton's law says that the pressure of a gas mixture is equal to the partial pressures of the combining gases. (CC BY-NC; CK-12)

Key Takeaway

The pressure of a gas in a gas mixture is termed the partial pressure.
Dalton’s law of partial pressure states that the total pressure in a gas mixture is the sum of the individual partial pressures.