3. A droplet of liquid A, of radius r1, is suspended in a stream of gas B. We postulate that there is a spherical stagnant gas film of radius r2 surrounding the droplet. The mole fraction of A in the gas phase is XA1 at r=r1 and xA2 at the outer edge of the film, r=r2. a. By a shell balance, show that for steady-state diffusion r2NAr is a constant within the gas film, and set the constant equal to r12NAr, the value at the droplet surface. b. Show that general flux equation and the result in (a) lead to the following equation for XA : r12NAr1=1xAcDABr2drdxA c. Integrate the equation obtained in (b) between the limits r1 and r to get NAr1=r2r1cDAB(r1r2)lnxB1xB2 What is the limit of this expression when r2