Benzene and naphthalene form an ideal solution over the entire range of composition. The vapour pressure of pure benzene and naphthalene at 300 K are 50.71 mm Hg and 32.06 mm Hg respectively. Calculate the mole fraction of benzene in vapour phase if 80 g of benzene is mixed with 100 g of naphthalene.

nbenzene =80g78gmol−1=1.026mol   nnaphthalene =100128=0.781mol      xbenzene =1.0261.026+0.781=0.568xnaphthalene =1−0.568=0.432  pbenzene =pbenzene ∘×xberzene                     =50.71×0.568=28.80mmpnaphthalene =32.06×0.432=13.85mmptotal =28.80+13.85=42.65mmybenzene  in vapour phase =xbenzene ×pbenzene ∘ptotal                                             =0.568×50.7142.65=0.675