$1($a$)$ Animonia gas is diffusing through a layer of stagnant air, $2\mathrm{.}5$ cm thick at a constant rate. Conditions are fixed so that the gas contains $50\%$ ammonia by volume at one boundary of the stagnant layer. The ammonia diffusing to the other boundary is quickly absorbed and its concentration is negligible at the plane. The temperature is $20$C and the pressure is atmospheric, under which conditions the value of D for ammonia$-$air system may be assumed as $0\mathrm{.}18$ cm$/$sec$.$ Calculate the rate of diffusion of ammonia through air layer in g moles$/$h cm R$-82\mathrm{.}06($cc$)($atm$)/$g mole K $($COL$.5$ marks