Multiple choice

A solution of urea in water has a vapour pressure of 266.3 torr. The vapour pressure of pure water is 355.1 torr. Calculate the mole fraction of urea in the solution.

  1. 0.25

  2. 0.50

  3. 0.75

  4. 1.0

  5. 1.25
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A Correct answer
Explanation

Correct answer. Urea is non-volatile. Water obeys Raoult’s law. Pwater = Xwater x P°water 266.3 torr = Xwater x (355.1 torr) Xwater = 266.3 / 355.1 = 0.75 From equation, X1 + X2 + X3 ...... = 1 Xurea + Xwater = 1 Xurea = 1 – 0.75 = 0.25

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