When a substance such as a gas tries to pass through a semi-permeable membrane, the rate of passage is measured by a value D known as diffusivity. The units for diffusivity are in cm^2/s. A formula for diffusivity is given by D = D_0exp(-E/RT). Here, D_0 is what's called a pre-exponential factor, E is the activation energy of the diffusion, R is a gas constant equal to 1.987 cal/mol.K, and T is the temperature in Kelvin.