What is Graham’s Law of Effusion?
Technically, it’s Graham’s Law of Effusion.
The molecules of a gas are in constant motion, colliding with each other and with the walls of the container. The average distance that a molecule travels between collisions (about 0.1 µm or 300 times the molecular diameter for N₂ at STP) is called its mean free path.
Graham’s Law deals with the rates at which gases escape through a small hole in the container. If the diameter of the hole is less than the mean free path of a molecule, the process is called effusion. If the diameter of the hole is greater than the mean free path of a molecule, the process is called diffusion.
In 1848, Thomas Graham found experimentally that the rate of effusion of a gas is inversely proportional to the square root of the mass of its molecules. This is now known as Graham’s Law of Effusion. We can write the formula as
Rate ∝ 1 √M
If we have two different gases with molar masses M₁ and M₂, the ratio of their rates of effusion is
Most frequently, you see the above formula for Graham’s Law of Effusion.
Here is a video on Graham’s Law of Effusion.