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Question:

The vapour pressure of pure liquids A and B are 400 and 600 mmHg, respectively at 298K. On mixing the two liquids, the sum of their initial volumes is equal to the volume of the final mixture. The mole fraction of liquids B is 0.5 in the mixture. The vapour pressure of the final solution, the mole fraction of components A and B in vapour phase, respectively are:

500 mmHg, 0.5,0.5

450 mmHg, 0.4,0.6

450 mmHg, 0.5,0.5

500 mmHg, 0.4,0.6

Solution:

Correct option is D. 500 mmHg, 0.4,0.6
P_total=X_A.P^0_A+X_B.P^0_B=0.5×400+0.5×600=500mmHg
Now, mole fraction of A in vapour,Y_A=P_A/P_total=0.5×400/500=0.4
and mole fraction of B in vapour,Y_B=1-0.4=0.6
Hence, the correct option is D