Explain how the molar mass of a gas affects the rate at which the gas diffuses and effuses

Diffusion  is defined as spreading of molecules of a substance, away from each other, throughout the space.

Effusion is the phenomena of escape of gas molecules through a very small orifice into an evacuated area. 

According to Graham's Law, rate of diffusion and effusion is inversely proportional to molecular mass.

Thus, with increase in molecular mass, rate of diffusion and effusion will decrease.