a spherical droplet of liquid a and radius r evaporates into a stagnant layer of gas b$.$ derive an expression for the evaporation rate of species a in terms of the saturation pressure of species a$,$ pa$($ro$)=$ pa$,$sat, the partial pressure of species a at an arbitrary radius r$,$ pa$($r$),$ the total pressure p$,$ and other pertinent quantities. assume the droplet and the mixture are at a uniform pressure p and temperature t$.$