Q. Vapour pressure of pure ‘A’ is 70 mm of Hg at $25^{\circ}C\ .$ It from 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
A) 28 mm
B) 56 mm
C) 70 mm
D) 140 mm
Explanation
(d)
$p = P_{A}^{\circ}X_{A} + P_{B}^{\circ}x_{B}$
$\Rightarrow \ \ \ 84 = 70\ \times \ 0.8 + P_{B}^{\circ} \times 0.2$
84=56+$P_{B}^{\circ} \times 0.2$
$P_{B}^{\circ}$ =$\frac{28}{0.2} = 140\ mm$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.