Effusion
The escape of gas molecules one at a time through a hole much smaller than the mean free path.
Formula / equation
rate ∝ 1 / √M
Unit
mol/s
In depth
Unlike diffusion, effusion involves no molecular collisions in the aperture, so the rate depends only on how often molecules hit the hole — that is, on their average speed. Measuring the time for a known volume to effuse is a classic method of finding molar mass.
Examples in the real world
A helium balloon deflating faster than an air-filled one; gaseous diffusion plants enriching uranium hexafluoride.