Gases :  Graham's Laws of Diffusion and Effusion
  • Only a few physical properties of gases depends on the identity of the gas. 
  • Diffusion - The rate at which two gases mix. 
  • Effusion - The rate at which a gas escapes through a pinhole into a vacuum. 
Thomas Graham


Graham's Law of Effusion

[Image] The time required for 25-mL samples
of different gasses to diffuse through a 
pinhole into a vacuum.

The Kinetic Molecular Theory and Graham's Law

Simplify the equation by multiplying both sides by two:

Rearrange to give the following:

Take the square root of both sides to obtain the following relationship between the ratio of the velocities of the gases and the square root of the ratio of their molar masses:

This equation states that the velocity (rate) at which gas molecules move is inversely proportional to the square root of their molar masses.

Next:  "Deviation from Ideal Gas Behavior: Van der Waals Equation"