A porous sphere of $3mm$ diameter is saturated with methanol. It is suddenly dropped into a

large tank of pure water at $15C.$ If it is assumed that the interdiffusion process is controlled

by the rate of diffusion within the sphere, calculate the time required for the mole fraction of

methanol at the center of the sphere to fall to $0\mathrm{.}01.$ The effective diffusivity through the

porous sphere is one$-$tenth of the ordinary molecular diffusivity $(30p).$