When
two gases are placed in contact, they mix spontaneously. This is due to the movement of molecules of one gas into the other gas. This process of mixing of gases by random motion of the molecules is called Diffusion. Thomas Graham observed that molecules with smaller masses diffused faster than heavy molecules.Diffusion law
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A light molecule diffuses quicker than a heavy molecule.
In 1829 Graham formulated what is now known as Graham's Law of Diffusion. It states that:
Under the same conditions of temperature and pressure, the rates of diffusion of different gases are inversely proportional to the square roots of their molecular masses.
Mathematically the law can be expressed as:
r1/r2 = M2/M1
where r₁ and r₂ are the rates of diffusion of gases 1 and 2, while M₁ and M, are their molecular masses.
When a gas escapes through a pin-hole into a region of low pressure of vacuum, the process is called Effusion.
The rate of effusion of a gas also depends, on the molecular mass of the gas.
Dalton's law when applied to effusion of a gas is called the Dalton's Law of Effusion. It may be expressed mathematically as
Effusion rate of Gas 1/Effusion rate of Gas 2 = M2/M1
(P, T constant)
The determination of rate of effusion is much easier compared to the rate of diffusion. Therefore, Dalton's law of effusion is often used to find the molecular mass of a given gas.
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