Graham's Law: Diffusion And Effusion - Practice Questions & MCQ

Graham’s Law of Diffusion
According to it "At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density or molecular weight". It is applicable only at low pressure.

Here r = rate of diffusion or effusion of a gas or liquid. M and d are the molecular weight and density respectively.

For any two gases, the ratio of the rate of diffusion at constant pressure and temperature can be shown as 


Hence diffusion or effusion of a gas or gaseous mixture is directly proportional to the pressure difference of the two sides and is inversely proportional to the square root of the gas or mixture effusing or diffusing out.

Some Other Relation Based on Graham’s law
As r = V/t = Volume/time, thus:


Here n represents the number of moles, w represents weight in gram and d represents distance travelled by a particular gas or liquid.

Differentiation Between Diffusion and Effusion

Diffusion
It is the movement of gascous or liquid molecules without any porous bars that is, spreading of molecules in all directions.

Effusion
It is movement of gases molecules or liquid molecules through a porous bar that is, a small hole or orifice.

Uses of Graham’s Law 

• Detecting the presence of Marsh gas in mines.
• Separation of isotopes by different diffusion rates. For example, U.235 and U-238
• Detection of molecular weight and vapour density of gases using this relation.