A common process for increasing the moisture content of air is to bubble it through a column of water. The air bubbles are assumed to be spheres having a radius of 1.0 mm, and are in thermal equilibrium with the surrounding water at 298 K. The vapor pressure of water is 0.03 atm at 298 K, and the total pressure of the gas inside the air bubble is 1.0 atm.

a. Draw a picture of the physical system, and state at least five reasonable assumptions for the mass-transfer aspects of the water evaporation process. What coordinate system should be used?

b. What are the simplified differential forms of Fick’s flux equation for water vapor (species A), and the general differential equation for mass transfer in terms of concentration c_A?