A pure liquid B moves upward slowly in a long tube through a film containing A . Then A slowly disappears by a first-order reaction (A --C) after it has dissolved. Assume that the velocity profile is approximately flat across the tube and only small amount of C formed during the process. Assume further that CA0 is the solubility of unreacted A in B. neglect temperature effects associated with the heat of reaction and consider mass transfer only. Develop the appropriate governing equation and boundary conditions needed to describe this mass transfer process at steady state.