Consider a liquid flowing down a vertical wall at a rate of \(1 \times 10^{-5} \mathrm{~m}^{2} / \mathrm{s}\) per meter unit width. Find the concentration at a height \(25 \mathrm{~cm}\) below the entrance for a dissolving wall such as a wall coated with benzoic acid as a function of perpendicular distance from the wall. Also find the local mass transfer coefficient.

Find the average mass transfer coefficient for a wall of height \(50 \mathrm{~cm}\). Assume \(D=2 \times\) \(10^{-9} \mathrm{~m}^{2} / \mathrm{s}\), and for other properties take those of water. Use \(C_{\mathrm{A}, \mathrm{s}}=20 \mathrm{~mol} / \mathrm{m}^{3}\) and the physical properties of water.