# What is Graham's Law of Effusion?

Jan 6, 2014

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

Rate₂/Rate₁ =√(M₁/M₂)

Most frequently, you see the above formula for Graham’s Law of Effusion.

Here is a video on Graham's Law of Effusion.