Derive an expression representing the concentration profile through a thin film $($of thickness $L),$ in steady state from the general equation of Fick's second law. Assume constant solute concentrations on both surfaces of the film, establishing a concentration gradient $C=(C2-C1),$ where $C2>C1.$

Write the expression to calculate the flux and illustrate in a drawing the concentration profile and the direction of the flow. Express the associated driving force and resistance.