mole fraction of He = mole of He/ total mole now mole of He = weight/M.wt = 2/4 = 0.5 mole of O2 = 5.99/32 = 0.187 total mole = 0.687 mole fraction of He = 0.5/0.687 = 0.727 Solution mole fraction of He = mole of He/ total mole now mole of He = weight/M.wt = 2/4 = 0.5 mole of O2 = 5.99/32 = 0.187 total mole = 0.687 mole fraction of He = 0.5/0.687 = 0.727.