A gas $A$ is diffusing at a constant rate through a layer of stagnant gas $B,1mm$ thick. Conditions are such that the gas contains $50$mol$\%A$ at one boundary of the stagnant layer. A diffusing to the other boundary is quickly absorbed and the concentration is negligible at that plane. The temperature is $295K$ and the pressure atmospheric $(101\mathrm{.}325$kPa$),$ and under these conditions the diffusivity of $A$ in $B,D_{AB}$ is $0\mathrm{.}18c\frac{m^2}{s}.$ Estimate the rate of diffusion of ammonia through the layer.