Graham's Law
Where;
r1 is the rate of effusion of the first gas,
r2 is the rate of effusion of the second gas,
M1 is the molar mass of the first gas,
M2 is the molar mass of the second gas.
Graham's law states that the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. Thus, if the molecular weight of one gas is four times that of another, it would diffuse through a porous plug or escape through a small pinhole in a vessel at half the rate of the other.