Diffusion through a stagnant film--alternate derivation in S18.2 an expression for the evaporation rate was obtained in Eq. 18.2-14 by differentiating the concentration profile found a few lines before. Show that the same results may be derived without finding the concentration profile. Note that at steady state, NAz is a constant according to Eq. 18.2-3. Then Eq. 18.2-1 can be integrated directly to get Eq. 18.2-14.