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Grahams Law of Effusion
Thomas Graham was an English Scientist. He proposed the Law of Effusion. According to Grahams
Law of Effusion
The rate of effusion of a gas is inversely proportional to the square root of its
density/molar mass at constant temperature and pressure”
Derivation
Rate of effusion 
Rate of effusion = 
Rate of effusion x
= 󰇛󰇜
The constant k is same for all gases, when they are all studied at the same temperature and
pressure.
Let us have two gases 1 and 2, having rate of effusion r
1
and r
2
and densities as d
1
and d
2
respectively.
According to Grahams Law
r
1
x
= k ……………..(2)
r
2
x
= k………………(3)
dividing and rearranging 2 & 3
Since, the density of a given gas is directly proportional to its molecular mass at constant
temperature and pressure grahams law of effusion can also be written as follows:
Where M
1
and M
2
are the molar masses of gases
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