Professor Graham investigated the relationship between the rate of diffusion of a gas and its molecular mass. He concluded from his investigation that the Rate of diffusion was inversely proportional to the square root of the molecular mass of the gas. Since volumes of any gas will be the same for the same moles at the same temperature and pressure, we can make a similar statement regarding the density of a gas. The Rate of diffusion of a gas is inversely proportional as to the square root of the density of the gas. This is a more useful formula when using an effusion chamber.
Effusion is defined as a special type of diffusion through a small hole or orific.
The molecular mass of an unknown gas can be determined by comparing it to a standard known gas of the same amount and under the same conditions of temperature and pressure. An effusion chamber is a partioned chamber which has a gas on one side and a relative vaccuum on the other side of the partition. There is a tiny hole called an orifice that separates the two sides. Suppose we wanted to determine the molecular mass of an unknown gas. The gas is inserted on one side of the chamber and is allowed to travel through the orifice to the vacuum side of the chamber until the pressure on the two sides are essentially the same.