Diffusion as a mass flux

Fick s Law of diffusion is normally expressed in molar units or  

The corresponding equation for component B indicates that there is an equal and opposite molar flux of that component. If each side of equation 10.4 is multiplied by the molecular weight of A. MA, then  

Although the sum of the molar concentrations is constant in an ideal gas at constant pressure, the sum of the mass concentrations is not constant, and dc /dy and dcg/dy are not equal and opposite, 

the diffusional process does not give rise to equal and opposite mass fluxes. 

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