DALTON'S LAW OF PARTIAL PRESSURES

Note: This important law is, for some reason, not part of the South African syllabus, and is only included here for further information. This topic is therefore unlikely be covered in South African textbooks, and it is even more unlikely that it would be examined.

1. Dalton's law of partial pressures

If a mixture of gases is confined in a container, each gas exerts its own pressure on the walls of the container, as if it alone were present. Its pressure, p, will be determined by the parameters of the gas equation, pv = nRT. The pressure thus exerted by a gas in a mixture of gases is known as the PARTIAL PRESSURE of the gas. This generalization is embodied in Dalton' s law:

Dalton's law of partial pressures
"The total pressure of a mixture of gases is the sum of the partial pressures of the constituent gases."

If a mixture of gases, A, B, C... is confined in a vessel at a given temperature, the pressure, ptot, of the mixture will be given by

where pA, pB, pC... are the partial pressures of the gases A, B, C... respectively.

This law is expected on the basis of the kinetic theory: in a mixture of gases, all gas molecules regardless of their composition, are free to move at random in the entire volume of the container, and thus collide with the walls of the container, thereby exerting pressure.

The law of partial pressures enables us to correct for water vapour in a gas, as shown in the following worked example.