a
\({P_{total}}\, = \,{X_A}\,P_A^o\, + \,{X_B}\,P_B^o\) \(\, = \,0.5\, \times \,400\,\, + \,0.5\, \times \,600\) \( = \,500\,mm\,Hg\)

Now , mole fraction of \(A\) in vapour

\({Y_A}\, = \,\frac{{{P_A}}}{{{P_{total}}}}\, = \,\frac{{0.5\, \times \,400}}{{500}}\, = \,0.4\) and mole fraction of \(B\) in vapour

\({Y_B}\, = \,1\, - \,0.4\, = \,0.6\)