An aqueous solution contains 20% urea by mass (molar masses: urea 60, water 18 g mol^-1). The vapour pressure of pure water at 30 degC is 31.8 mmHg. Treating urea as non-volatile, the vapour pressure of the solution in mmHg is

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An aqueous solution contains $20%$ urea by mass (molar masses: urea $60$ , water $18 g mo l^{- 1}$ ). The vapour pressure of pure water at $3 0^{\circ} C$ is $31.8 mmHg$ . Treating urea as non-volatile, the vapour pressure of the solution in mmHg is

Take

100 g

of solution: urea

20/60 = 0.3333 mol

, water

80/18 = 4.4444 mol

. Then

x_{1} = 4.4444/ (4.4444 + 0.3333) = 0.9302

and

p = x_{1} p^{0} = 0.9302 \times 31.8 = 29.58 mmHg

. Using mass fraction instead of mole fraction is the usual mistake.