Gases with lower masses diffuse more rapidly than gases with higher molar masses because lighter molecules move faster on average than heavier ones. The rate of diffusion of the gas is inversely proportional to the density of gas.
In the mid-1800s, Thomas Graham experimented with effusion and discovered a very significant relationship: “lighter gas molecules will travel faster than heavier gas molecules”.
So, assuming the temperature and pressure remain constant, atoms or molecules with a lower molecular mass will effuse faster than atoms or molecules with a higher molecular mass. Graham even went further by finding out how much faster one molecule would be over another.
This became known as Graham's Law, and it states that the effusion rate of a gas is inversely proportional to the square root of its molecular mass. Usually this formula is used when comparing the rates of two different gases at equal temperatures and pressures. The ratio of the rate (or speed) of gas A over gas B is equal to the square root of the mass of gas B over the mass of gas A.
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