A.    One method of measuring diffusion coefficients of vapours is to measure the rate of evaporation of a liquid in narrow tubes. In one such experiment, a glass tube 1 cm in diameter was filled with water at 20°C to within 4 cm of the top. Dry air at 20°C and 1.013 bar was blown across the top of the tube. At the end of 24 hours of steady-state operation, the level of the water dropped 0.1 cm. Calculate the diffusivity of the air-vapour system at 20°C.

B.    Nitrogen diffuses steadily through a stagnant layer of air which is 0.5 cm thick. The concentration of N2 is 0.08 kg/m3 at one face and zero at the other. The total pressure is 1 atm and the temperature 20°C. Calculate the time required for 1 kg of N2 to diffuse across 1 sq. m. of this air film.