The mean free path of gas molecule is proportional to x^th power of number density and y^th power of diameter of molecule. Here x and y are 

Concept:

• The average distance between two successive collisions is called the mean free path.
• The actual distance a particle, such as a molecule in a gas, moves before a collision, known as a free path,
• It cannot generally be given because its calculation would require knowledge of the path of every particle in that region.
• Hence the distance that a particular gas molecule travels without colliding is known as the mean free path.

Explanation:

• The mean free path

    or, 

•  where  
•  
•   

Hence the value of the  x^th power of number density = -1,  yth power of the diameter of molecule = -2