Molecular diffusion occurs in any homogeneous phase i$.$c$.$ gas or liquid or solid phases.

A$)$ What is molecular diffusion? $[2$ marks$]$

B$)$ Compare and contrast pore diffusion and Knudsen diffusion $[4$ marks$]$

C$)$ Outline two $(2)$ reasons why effective diffusivity in a porous matrix is less than diflusivity in a free fluid $[2$ marks$]$

D$)$ Butanol $C_4{H}_{4}OH($A$)$ diffuses across a film of non$-$diflusing water $H_{2}O($B$)$ at $20C.$ The lilm is $0\mathrm{.}1cm$ and butanol concentration on opposite sides of the film are $10\%$ and $4$@ by weight respectively. The diffusivity of butanol in water solution at $20C$ is $5\mathrm{.}9{10}^{-5}c\frac{m^2}{sec}.$ Calculate the steady$-$state mass flux of butanol $[10$ marks$]($Data: Density. solution