Assertion: A lighter gas diffuses more rapidly than a heavier gas. Reason: At a given temperature, the rate of diffusion of a gas is inversely proportional to the density.

Rate of diffusion of gases is inversely proportional to the square root of molecular mass as well as with their densities. 1 r d where d is density Thus, a lighter gas will diffuse more rapidly than a heavier gas because of this. Therefore, the correct answer would be (a).