JEE Main 2021 · Chemistry · Solutions, Solid State and Surface Chemistry · hard

Liquids P and Q form an ideal solution with vapour pressures of the pure liquids 300 torr and 100 torr respectively. A liquid solution containing 20…

Numerical answer

Liquids P and Q form an ideal solution with vapour pressures of the pure liquids

300\ \mathrm{torr}

and

100\ \mathrm{torr}

respectively. A liquid solution containing

20

mol% P is brought to equilibrium with its vapour; the vapour is completely condensed to a new liquid, which is again brought to equilibrium with its own vapour. The mole percentage of P in this second vapour, to the nearest integer, is

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Answer: 69

First equilibrium:

P_{\text{tot}}=0.2(300)+0.8(100)=140

torr and

y_P=\dfrac{60}{140}=0.4286

. The condensed liquid therefore has

x_P=0.4286

. Second equilibrium:

P_{\text{tot}}=0.4286(300)+0.5714(100)=128.57+57.14=185.71

torr, and

y_P=\dfrac{128.57}{185.71}=0.6923

. As a percentage this is

69.2\%

, so the nearest integer is

69

. Each condensation step enriches the more volatile component.

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