Consider a sphere of porous material of radius R surrounded by a gas having mole fraction XA0 (or, if you prefer, CA0) the bulk of the gas.
Derive the differential equation that when solved will give the mole fraction of gas A (XA) [or the concentration of gas A, CA] at steady state as a function of radial position in the sphere.
Write the boundary conditions necessary to solve the problem.
Solve the differential equation in a) to find mole fraction (or concentration) of gas A as a function of radial position.