Helium gas is stored at 20*C under a pressure of 5x10^5 Pa in a glass cylinder 50 cm in diameter and 10cm long. The glass walls are 3 mm thick. Overtime the Helium will gradually diffuse out of the cylinder. (The diffusion coefficient of He through oxide glass at 20*C is about 4x10^?4 m2/s and the concentration of He on the inner surface is about 2.2x10^?3kmol/m3).
a) Sketch the concentration profile of helium gas through the thickness of the cylinder walls.
b) What is the initial rate of mass loss through the cylinder? Hint: Remember to include the flux through the top and bottom surfaces.