Graham's Law of Diffusion
When two gases are placed in contact, they mix
spontaneously. This is due to the movement of molecule of one gas into the
other gas. This process of mixing of gases by random motion of the molecules is
called as diffusion.
In 1829, Graham formulated what is now known as Graham's law of
diffusion. It states that,
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 = root of ( M1/ M2 )
where r1 and r2 are the rates of diffusion of gases 1 and 2, while M1
and M2 are their molecular masses respectively.
When a gas escapes through a pin-hole into a
region of low pressure or vacuum, the process is called Effusion. The rate of diffusion 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
rate of Gas 1/ Effusion rate of Gas 2 =
root of (M2/ M1)
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.
gas A effuses through a pin hole in 73 seconds. The same volume of SO2 under
identical conditions effuses in 75 seconds. Calculate the molecular mass of A.
If a gas diffuses at the rate of one quarter as
fast as N2. Find the molecular mass.