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 intorr is

• A. 50
• B. 25
• C. 37.5
• D. 53.5

Answer 1

Answer: (A)

Molar mass of $C_6H_6 $ = 78 g $mol^{-1}$
Molar mass of $C_6H_5-CH_3 $ = 92 g $mol^{-1}$
$ n$ (Benzene) = 78/78 = 1
$n$ (Toluene) = 46/92 = 0.5
$x$ (Benzene) = $\frac{1}{1+0.5} = \frac{10}{15} = \frac{2}{3}$
$p$ (Benzene) = $x$ (Benzene) $\times \, p^\circ $ (Benzene)
= $\frac{2}{3} \times 75$ = 50 torr.