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  4. Vapour pressure of pure 'A' is 70 mm of Hg at 25° C. It forms an ideal solution with 'B' in which mole fraction of A is 0.8. If the vapour pressure of the solution is 84 mm of Hg at 25°C, the vapour pressure of pure 'B' at 25°C is

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Q. Vapour pressure of pure $'A'$ is $70\, mm$ of $Hg$ at $25^\circ \,C$. It forms an ideal solution with $'B'$ in which mole fraction of $A$ is $0.8$. If the vapour pressure of the solution is $84\, mm$ of $Hg$ at $25^\circ$C, the vapour pressure of pure $'B'$ at $25^\circ$C is

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Solution:

$p=p_{A}^{\circ} x_{A}+p_{B}^{\circ} x_{B}$

$\Rightarrow \,\,\,\,\, 84=70 \times 0.8+p_{B}^{\circ} \times 0.2$

$84=56+p_{B}^{\circ} \times 0.2$

$p_{B}^{\circ}=\frac{28}{0.2}=140\, mm$