What Is Dalton's Law of Partial Pressures?

Dalton's law of partial pressures is used to determine the individual pressures of each gas in a mixture of gases.

The total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases.

Pressure_Total = Pressure_{Gas 1} + Pressure_{Gas 2} + Pressure_{Gas 3} + ... Pressure_{Gas n}

An alternative of this equation can be used to determine the partial pressure of an individual gas in the mixture.
If the total pressure is known and the moles of each component gas are known, the partial pressure can be computed using the formula:

P_x = P_Total ( n_x / n_Total )

where:

P_x = partial pressure of gas x P_Total = total pressure of all gases n_x = number of moles of gas x n_Total = number of moles of all gases

This relationship applies to ideal gases but can be used in real gases with very little error.

Dalton's law is an ideal gas law. It is only an approximation for real gases. The deviation from the law increases with increasing pressure. At high pressure, the volume occupied by a gas becomes significant when compared to the free space between particles. At high pressure, intermolecular forces between particles become more of a consideration.

• Dalton, J. (1802). "Essay IV. On the expansion of elastic fluids by heat." Memoirs of the Literary and Philosophical Society of Manchester, vol. 5, pt. 2, pp. 595–602.
• Silberberg, Martin S. (2009). Chemistry: the molecular nature of matter and change (5th ed.). Boston: McGraw-Hill. p. 206. ISBN 9780073048598.