DALTON’S LAW OF PARTIAL PRESSURES

Dalton’s law states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of all the gases in the mixture. Each partial pressure is the pressure that the gas would exert if the other gases were not present.

This law is particularly useful when gases are collected in vessels above the surface of an aqueous or nonaqueous solution. In an aqueous solution, the partial pressure of water vapor may be subtracted from the total pressure to help determine the partial pressures of the other gases or vapors present. The percent composition of a gaseous mixture may then be calculated, based on Dalton’s law. To calculate the composition of the solution phase (e.g., in mole fraction or mole percent), Raoult’s Law (see equation 1.15) must be used.

GRAHAM’S LAW OF EFFUSION

Recall that kinetic molecular theory states that the average speed of molecules in motion can be approximated by the root-mean-square speed, Urmd

Diffusion is the migration and mixing of molecules of different substances as a result of a concentration gradient across a fixed space and random molecular motion. Effusion is the escape of gas molecules of a single substance through a tiny orifice (pinhole) of a vessel holding the gas. Graham’s law of effusion states that the rates of effusion of two different gases escaping, A and B, are inversely proportional to the square roots of their molar masses:

Note that, at a fixed temperature, the effusion time is inversely proportional to the effusion rate, while the mean distance traveled, as well as the amount of gas effused, is directly proportional to the effusion rate.