Question

Benzene and toluene form nearly ideal solutions. At 20$^{\circ}$ 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

Answer 1

Answer: (D)

Mixture contains 78g 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 $p^{\circ}_{b} = 75$ torr
$\therefore $ partial vapour pressure of benzene $= p^{\circ}_{b}X_{b}$
$= 75\times\frac{2}{3} = 50$ torr