Hydrogen at a pressure of 2 atm flows within a tube of diameter 40 mm and wall thickness 0.5 mm. The outer surface is exposed to a gas stream for which the hydrogen partial pressure is 0.1 atm. The mass diffusivity and solubility of hydrogen in the tube material are 1.8 x 10^-11 m²/s and 160kmol/m³ atm. respectively. When the system is at 500 K, what is the rate of hydrogen transfer through the tube per unit length (kg/s ∙ m)?