please clarify mass fraction showing nothing. What is nothing, is it zero or do you mean something else? From your plot, mass fraction on the wall is 0.2, right? So it has a constant non-zero distribution. Based on the information provided, it's impossible to say whether this is correct or not, I'll let you dig deepre into this. nBack to the mole fraction distribution, denoting mass fraction as w_i, mole fraction x_i will benx_i =(w_i/M_i)/(SUM_j (w_j/M_j)nwhere M_j are respective molecular weights of species. So if you have more than two species and other species distributions vary along the wall, then the denominator is not necessarily constant along the wall and mole fraction of H2O doesn't have to be constant when the mass fraction is constant.