Question

Oxygen diffuses through a stagnant layer of air, 1mm thick, ambient temperature 28°C and 1atm total pressure. The partial pressure of oxygen on two sides of layer is P1=0.9atm and P2=0.1atm respectively. Calculate the value of Molar flux (in mol/m^2.s) with respect to an observer moving with molar average velocity. Calculate this value at the end of the path where P2=0.1atm. Use R=8.205*10^-5m^3atmK^-1mol^-1.

(a) 2.713

(b) 1.713

(c) 0.1713

(d) 0.01713

I got this question in an online quiz.

This key question is from Diffusion in a Binary Gas Mixture in chapter Diffusion in Solids and Molecular Diffusion in Fluids of Mass Transfer

Answer 1

The correct choice is (b) 1.713

Easy explanation: This is a case of diffusion of A through non-diffusing B.

                          Molar flux of oxygen, NA= (DAB*P ln⁡[(P-P2)/(P-P1)])/RTl

                          Putting all the required values in the above equation.

                                             NA=1.904 mol/m^2.s

                           Molar average velocity, U= (NA+NB)/C

                                                                      U=NA/C                        (NB=0)

                                                                      U=NART/P

                                                                      U=4.702*10^-2m/s

                                                                      Now, UA=UC/CA

                                                                                                              UA=U/yA=0.4702 m/s (yA is mole fraction of A at the end)

                                                      JA=CA(uA-U)

                                                          =PA*( uA-U)/RT

                                                          =1.713mol/m^2.s.