Given, \(P^o_A\,=\,74.7\,torr\), \(P^o_B\,=\,22.3\,torr\)
\(n_{benzene}\,=\,1.5\,mol\), \(n_{toluene}\,=\,3.5\,mol\)
\(n_{solution}\,=\,1.5+3.5=5\,mol\)
\(X_A\,=\,\frac {n_{benzene}}{n_{solution}}\,=\,\frac {1.5}{5}\)
Total \(V.P.\) of solution
\( = \left( {\frac{{1.5}}{5}\, \times \,74.7\, + \,\frac{{3.5}}{5}\, \times \,22.3} \right)\,torr\)
\(=\,(15.6+22.4)\,torr\,\,=\,38\,torr\)
Mole fraction of benzene in vapour form
\(=\,\frac {22.4}{38}\,=\,0.589\)