Benzene and toluene form nearly ideal solutions

• Solutions

Benzene and toluene form nearly ideal solutions. At 20°C, the vapour pressure of benzene is 75 torr and that of toluene is 22 torr. The partial vapour pressure of benzene at 20°C for a solution containing 78 g of benzene and 46 g of toluene in torr is

1. 25
2. 37.5
3. 50
4. 53.5

Answer

According to Raoult's law, P_B = P°_B X_B

P°_B = 75 torr

X_B = (78/78)/(78/78+46/92) = 1/1.5

P_B = 75 x 1/1.5 = 50 torr

The correct option is C.