Q. Benzene and toluene form nearly ideal solutions. At $25^{\circ}C,\$ the vapour pressure of benzene is 75 torr and that of toluene is 22 torr. The partial vapour pressure of benzene at $20^{\circ}C$ for a solution containing 78 g of benzene and 46 g of toluene in torr is
A) 53.5
B) 37.5
C) 25
D) 50
Explanation
(d)
Mixture contains 78 g benzene = 1 mole benzene and 46 g toluene = 0.5 mole toluene
Total mole of benzene and toluene=1.5 mol
Mole fraction of benzene in mixture
=$\frac{1}{1.5} = \frac{2}{3}$
VP of benzene $p_{b}^{\circ}$ =75 torr
$\therefore$ Partial vapour pressure of benzene = $p_{b}^{\circ}\ X_{b}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 75\ \times \frac{2}{3} = 50\ torr\$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.