Q. 94.1( 48 Votes )

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.

Answer :


Vapour pressure of water, PIo = 23.8 mm of Hg

Weight of water, w1 = 850 g

Weight of urea, w2 = 50 g

Molecular weight of water, M1 = 18 g/mol

Molecular weight of urea, M2 = 60 g/mol

n1 = = 47.22 mol

n2 = = 0.83 mol

We have to calculate vapour pressure of water in the solution p1

By using Rault's therom,

(relative lowering)

PI = 23.4 mm of Hg


The vapour pressure of water in the solution is 23.4mm of Hg and its relative lowering is 0.0173.

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