# Graham's Law of Effusion, Gases Diffusion and Mean Free Path Average

## Graham's Law of Effusion

The rate of effusion can be quantified.
• Consider two gases with molar masses M1 and M2,
• since the rate of effusion (r) is directly proportional to (u),
• where u) is given by the equation: u =  3RT/M
• The relative rate of effusion (r1/r2) will be equal to  M2/M1

### Exercises on Graham's Law of Effusion

Exercise on Calculating the Ratio of the Effusion Rates

Check your answers here: Solution to the Exercise on Calculating the Ratio of the Effusion Rates

## Gases Diffusion and Mean Free Path Average

Definition:
"Diffusion of a gas is the spread of the gas through space".

Diffusion is thus faster for light gas molecules.

Diffusion is slowed by gas molecules colliding with eachother. Because of these collisions of the gas molecules, their direction of motion is always changing.

Average distance traveled by a gas molecule between collisions is called "mean free path".

### Example

The mean free path of gas molecules at about 100 km altitude is about 106 times that at earth's surface.