b
\({P_A}\, = \,P_A^o\, \times \,{x_A}\, = \) total pressure \(\times y_A\)

\({P_B}\, = \,P_B^o\, \times {x_B}\, = \)  total pressure \(\times y_B\)

where \(x\) and \(y\) represents mole fraction in liquid and vapour phase repsectively.

\(\frac{{P_B^o{x_B}}}{{P_A^o{x_A}}}\, = \,\frac{{{y_B}}}{{{y_A}}};\)

\(\frac{{P_B^o(1 - \,{x_A})}}{{P_A^o\,{x_A}}}\, = \,\frac{{1 - \,{y_A}}}{{{y_A}}}\)

on putting values \(\frac{{119\,(1 - \,0.50)}}{{37\, \times \,0.50}}\, = \,\frac{{1 - \,{y_A}}}{{{y_A}}}\)

on solving \(y_A\,=\,0.237\)