$2($a$)$ Distinguish the terms molecular diffusion and bulk flow.

$($b$)$ A small diameter tube closed at one end was filled with acetone to within $18mm$ of the top and maintained at $290K$ with a gentle stream of air blowing across the top. After $15000sec,$ the liquid level was fallen to $27\mathrm{.}5mm,$ the vapour pressure of acetone was $21\mathrm{.}95$kPa and atmospheric pressure was $99\mathrm{.}75$kPa.

Calculate the diffusivity of acetone in air given the density of acetone is $790k\frac{g}{m^3}$ and the molecular weight of acetone is $58k\frac{g}{k}$mol.

$($c$)$ In an air$-$carbon dioxide mixture at $298K$ and $202\mathrm{.}6$kPa, the concentration of $C{O}_2$ at two planes $(3mm)$ apart are $15$ vol. $\%$ and $25$ vol.$\%.$ The diffusivity of $C{O}_2$ in air at $298K$ and $202\mathrm{.}6$kPa is $8\mathrm{.}2{10}^{-6}\frac{m^2}{s}.$

Calculate the rate of transfer of $C{O}_2$ across the two planes, assuming: $a.$ Equimolecular counter diffusion. b$.$ Diffusion of $C{O}_2$ through a stagnant air layer.

$($d$)$ In several important processes, one component in a gaseous mixture will be transported relative to a fixed plane, such as a liquid interface, for example, and the other will undergo no net movement. In gas absorption a soluble gas $A$ is transferred to the liquid surface where it dissolves, whereas the insoluble gas $B$ undergoes no net movement with respect to the interface. Similarly, in evaporation from a free surface, the vapour moves away from the surface, but the air has no net movement. This is often referred to as stagnant diffusion $($i$.$e$.,$ Mass transfer through a stationary second component$)$

Ammonia gas is diffusing at a constant rate through a layer of stagnant air $1mm$ thick. Conditions are such that the gas contains $50$ percent by volume ammonia at one boundary of the stagnant layer. The ammonia diffusing to the other boundary is quickly absorbed and the concentration is negligible at that plane. The temperature is $295K$ and the pressure atmospheric, and under these conditions the diffusivity of ammonia in air is $0\mathrm{.}18c\frac{m^2}{s}.$

Estimate the rate of diffusion of ammonia through the layer.

$(5)$

$($e$)$ Provide well labelled diagrams drawn by hand only of each of the five types of fuel cells that you are now familiar with.

$($f$)$ With the aid of a well labelled diagram drawn by hand only and briefly discuss what strategy to use in reducing mass transfer effects on polymer electrolyte fuel cell efficiency