The rate of diffusion of a gas is inversely proportional to both the pressure of the gas and the square root of its molecular mass. This relationship is quantitatively expressed in Graham’s Law of Effusion. According to this law, the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molecular mass, mathematically expressed as:

Where R is the rate of diffusion or effusion and M is the molecular mass of the gas.

The rate of diffusion is also influenced by the pressure of the gas. At higher pressures, the rate of diffusion decreases. However, the relationship between gas pressure and the rate of diffusion is not as straightforward as the relationship with molecular mass, and it isn't as commonly discussed in simple terms as is Graham’s Law. Typically, the effect of pressure on the rate of diffusion is more significant in specific contexts, such as under varying atmospheric conditions.

In fundamental terms, a higher pressure implies more gas molecules are present, leading to increased collision frequency among them, which can slow down the overall rate at which individual molecules spread out or move through a medium.