Question

What is the vapour pressure of pure liquid A at 27C if it forms an ideal solution with another liquid B,the vapour pressure and mole fraction of pure liquid B at 27C is 140 torr and 0.2 respectively.Total vapor pressure of solution is 84 torr at 27C.

Answer 1

Dear Student,

$$\begin{gathered} \mathrm{According}\mathrm{to}\mathrm{Raoult}\text{'}\mathrm{s}\mathrm{law}, \\ {\mathrm{P}}_{\mathrm{T}}={\mathrm{P}}_{\mathrm{A}}°{\mathrm{X}}_{\mathrm{A}}+{\mathrm{P}}_{\mathrm{B}}°{\mathrm{X}}_{\mathrm{B}} \\ {\mathrm{X}}_{\mathrm{B}}=0.2 \\ \mathrm{So},{\mathrm{X}}_{\mathrm{A}}=1-0.2=0.8 \\ \mathrm{Now},84={\mathrm{P}}_{\mathrm{A}}°\times 0.8+140\times 0.2 \\ \mathrm{or}{\mathrm{P}}_{\mathrm{A}}°=70\mathrm{torr} \end{gathered}$$