a) A membrane can separate gas mixture because different gases have different permeability through the membrane. The permeate flux across unit membrane area under unit pressure gradient is called permeability and the ratio between the permeability of gas A and that of gas B is defined as selectivity of A to B. Membrane separation process depends upon five separation mechanisms including molecular sieving, Knudsen diffusion, solution diffusion, surface diffusion, and capillary condensation. Using a schematic diagram, elaborate on the mechanism of surface diffusion and solution diffusion in membrane separation