$11$ An ethanof $($A$)-$water $($B$)$ sotution in the form of a stagnant firm $2\mathrm{.}5mm$ thick at $30C$ is $m$ contact with each other at one surface. The concentration of ethanofon bott the ends are $15\mathrm{.}0$ wt and $3\mathrm{.}0$ wt $\%$ respectively and the comesponding densites ase $963k\frac{g}{m^3}$ and $981k\frac{g}{m^3}$ respectively. The diffurivity of ethanot is

$0\mathrm{.}725{10}^{-9}\frac{m^2}{s}.$ calculate the flax assuming steadystate operation.