JEE Main · 2005 · 08 May 2005 · Offline · Solutions

Benzene and toluene form nearly ideal solutions. At

2 0^{\circ}

C, the vapour pressure of benzene is 75 torr and that of toluene is 22 torr. The partial vapour pressure of benzene at

2 0^{\circ}

C for a solution containing 78 g of benzene and 46 g of toluene in torr is

(A)50✓ Correct
(B)25
(C)37.5
(D)53.5

Answer: (A)

Solution

Benzene and toluene form a nearly ideal pair, so Raoult's law applies and the partial pressure of benzene is

P_{B} = P_{B}^{\circ} X_{B}

. The

78

g of benzene is exactly one mole (

M = 78

) and the

46

g of toluene is half a mole (

M = 92

), giving a benzene mole fraction

X_{B} = \frac{1}{1 + 0.5} = \frac{2}{3}

. With

P_{B}^{\circ} = 75

torr, the benzene partial pressure is

75 \times \frac{2}{3} = 50

torr.

JEE Main · 2005 · 08 May 2005 · Offline · Single correct option

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