Effusion

Effusion: the escape of a gas through a small opening.

Graham’s Law of Effusion:  molecules move and effuse according to the formula:

In plain English, this means lighter molecules move faster and heavier molecules move more slowly.  If you have a small opening, such as a leak in a tank, the lighter molecules will escape faster.

For the purpose of these calculations, the mass of a molecule is its molar mass, which is simply the same number of grams as the molecule’s atomic mass.