Inside a Stefan tube with a diameter of 20 mm, there is water with a depth of 5 mm at 20C. The total pressure is 91.19 kPa, and the surrounding environment is dry air. The water surface is covered with a stagnant air film with a thickness of 15 mm. The concentration of water in the stagnant air film can be neglected. The vapor pressure of water at 20C is PA = 2.33 kPa, and the diffusion coefficient at 25.9C and 101.325 kPa is 0.258 cm2/s.

a. Calculate the rate of water evaporation

. b. Determine how much water will evaporate after 15 minutes.