Question 3 (3 marks) Methane (CH4) is diffusing in a straight tube 0.1 m long containing Helium (He) at 298K and a total pressure of 1.013 ~ 10% Pa. The partial pressure of CH4 is 2.24 x 104 Pa at one end and 6.6x103 Pa at the other end. Helium is insoluble in one boundary and hence non-diffusing. Diffusivity of CH4-He pair at 1.013 ~ 10Pa and 298 K is 0.675 cm/s. (a) Calculate the mass flux of CH4 in He. (2 marks) (b) Do the same as (a) but assume that He also diffuses; that is, both boundaries are permeable to both gases and the flux is equimolar counterdiffusion. In which case is the flux higher? (1 mark)