The Total pressure observed by mixing two liquid A   and   B is 350   mm   Hg when their…

Let Vapour pressure of pure $\mathrm{A}$ be ${\mathrm{P}}_{\mathrm{A}}^{\mathrm{o}}$

Let Vapour pressure of pure $\mathrm{B}$ be ${\mathrm{P}}_{\mathrm{B}}^{\mathrm{o}}$

Using Raoult's law and Dalton's law:

$$\begin{gathered} \mathrm{When} {\mathrm{X}}_{\mathrm{A}} = 0.7 \& {\mathrm{X}}_{\mathrm{B}} = 0.3 \\ {\mathrm{P}}_{\mathrm{s}} = 350 \\ \Rightarrow {\mathrm{P}}_{\mathrm{A}}^{\mathrm{o}} \times 0.7 + {\mathrm{P}}_{\mathrm{B}}^{\mathrm{o}} \times 0.3 = 350 ....\left(\mathrm{i}\right) \\ \mathrm{When} {\mathrm{X}}_{\mathrm{A}} = 0.2 \& {\mathrm{X}}_{\mathrm{B}} = 0.8 \\ {\mathrm{P}}_{\mathrm{s}} = 410 \\ \Rightarrow {\mathrm{P}}_{\mathrm{A}}^{\mathrm{o}} \times 0.7 + {\mathrm{P}}_{\mathrm{B}}^{\mathrm{o}} \times 0.3 = 350 ....\left(\mathrm{ii}\right) \\ \mathrm{Solving} \left(\mathrm{i}\right) \mathrm{and} \left(\mathrm{ii}\right) \\ {\mathrm{P}}_{\mathrm{A}}^{\mathrm{o}} = 314 \mathrm{mm} \mathrm{Hg} \\ {\mathrm{P}}_{\mathrm{B}}^{\mathrm{o}} = 434 \mathrm{mm} \mathrm{Hg} \end{gathered}$$