18.1-1. diffusion of methane through helium. a gas of ch4 and he is contained in a tube at 101.32 kpa pressure and 298 k. at one point, the partial pressure of methane is pa1 = 60.79 kpa, and at a point 0.02 m distance away, pa2 = 20.26 kpa. if the total pressure is constant throughout the tube, calculate the flux of ch4 (methane) at steady state for equimolar counterdiffusion.