Adrop of liquid a of radius r1 is suspended in a stream of gas b. there is a spherical stagnant gas film of radius r2 surrounding the droplet. the concentration of a in the gas phase is xa1 at r=r1 and xa2 at the outer edge of the film, r=r2.a) integrate the shell balance to derive the expression for the mole fraction of a in the stagnant film. b) derive an expression for the flux of a at the gas-liquid interface (r=r1).