Graham’s Law of Diffusion states that provided the temperature and pressure are constant, that the rate of diffusion of gas is inversely proportional to the square root of its density.

R ∝ 1/√e

R_{1}R_{2 }= √(e_{2}/e_{1})

Since relative molecular mass = 2 x density

i.e M = 2e

R_{1}R_{2 }= √(M_{2}/M_{1})