I added it below

Mass transfer in a solid is described as o'c (Fick's law). Here, concentration profile is c=c(x.t) and effective diffusion coefficient is Dem. For a slab, plate or plane wall; solve this partial differential equation for average concentration (c) according to initial and boundary conditions below. Explain if you can use error function approach for this solution or you can't, with reasons. Find the solution. ac Boundary conditions: = 0 (Symmetry) ax x lx=0,1 C(x =L, t) = Cs (Surface concentration, stated) Initial conditions: C(x, t= 0) = C Here; C = Constant initial concentration Cs: Constant concentration on both surfaces of the plane wall for t20 Ci C Cav CF X = -L X = 0 X = L