.Water is being evaporated from a plane surface into the surrounding still air.The water layer is 1 mm thick and temperature of air and water are 300K.Total pressure is 101.3 kPa.The absolute humidity of air is 0.002 kg water per kg dry air.The molecular diffusion of vapour is assume to take place through a gas film 5mm thick.Diffusivity of water vapour in air is 2.6x10-5m2/s at 300K and vapour pressure of water at this temperature is 3.2kPa.Calculate the time required for complete evaporation.