# Does the size of a hole in a container affect the rate of effusion?

Graham's Law of Effusion states that the rate of effusion of a gas is inversely proportional to the square root of density of the gas.
If there were two containers both with the same gas but one of the containers had a larger hole than the other, would the rate of effusion be quicker for the gas in the container with the larger hole?

## 1 Answer

A small hole is used. Knudsen conducted experiments similar to Graham's but approximately 50 years later (in 1909) and used a 0.025 mm diameter hole in a platinum strip of 0.0025 mm thickness. It was observed the size was not critical as long as the mean free path of the gas was ten times larger than the hole, thus the gas was at low pressure.

In these experiments the gas is allowed to stream out of a small hole in the vessel into vacuum. The velocity of the escaping gas is just that of the trapped gas, that would be obtained if the hole was not there, and these velocities would be the molecular velocities. Thus the size of the hole must not disturb the distribution of molecular velocities, achieved by being smaller than mean free path.