Graham's law
Graham's law, sometimes known as Graham's law of diffusion, or Graham's law of effusion, states that the rates at which gases diffuse are inversely proportional to the square roots of their relative molecular masses.
For example, helium (monatomic) has a relative molecular mass of 4, and oxygen, in its normal diatomic form O2, has a relative molecular mass of 32. The ratio oxygen/helium of 32/4 = 8 leads to the prediction by Graham's law that helium will diffuse about 2.8 times (the square root of 8 is 2.83) as quickly as oxygen.
In practice diffusion is complicated by the fact that two gases are always involved. Graham's law applies more accurately to effusion, in which a gas escapes through a small hole in its container into a vacuum.
The law was first proposed by Thomas Graham (1805-1869), a Scottish scientist who became Professor of chemistry at Glasgow University.
The effect is exploited in the separation of the isotopes of uranium by the repeated diffusion of the gaseous uranium hexachloride.