Given PA∘=90mmHg, at 25∘C
PB=∘15mmHg
and \begin{matrix}{X}_{A}=0.6 \\ {X}_{B}=0.4\end{matrix}}{P}_{T}={X}_{A}{P}_{A}^{o}+{X}_{B}{P}_{B}^{o}
=(0.6×90)+(0.4×15)
=54+6=60mm
Now mol fraction of B in the vapour phase
i.e. YB=PTPB=60XBPBo=0.1=1×10−1
therefore: x=1