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Vapour pressure of pure water at 298 K is 23.8 mm Hg. 50 g of urea (NH2CONH2) is dissolved in 850 g of water. Calculate the vapour pressure of water for this solution and its relative lowering.

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1 = 23.8 mm 

W2 = 50 g 

M2 (urea) = 60 g mol-1 

W1 = 850g 

M1 (Water) = 18g mol-1 

Here we have to calculate Ps 

Applying Raoult’s law,

Thus, relative lowering of vapour pressure = 0.017 

Substituting P° = 23.8 mm Hg


We get, 

23.8 – Ps = 0.017 Ps 

Ps = 23.4 mm Hg 

Thus, vapour pressure of water in the solution = 23.4 mm Hg

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