Page "Graham's law" Paragraph 11

Perhaps the greatest success of the kinetic theory of gases, as it came to be called, was the discovery that for gases, the temperature as measured on the Kelvin ( absolute ) temperature scale is directly proportional to the average kinetic energy of the gas molecules.

The kinetic energy of any object is equal to one-half its mass times the square of its velocity.

Thus, to have equal kinetic energies, the velocities of two different molecules would have to be in inverse proportion to the square roots of their masses.

The rate of effusion is determined by the number of molecules entering an aperture per unit time, and hence by the average molecular velocity.

Graham's law for diffusion could thus be understood as a consequence of the molecular kinetic energies being equal at the same temperature.