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Question 9

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

It is given that vapour pressure of water,PIo = 23.8 mm of Hg

Weight of water taken, w1= 850 g

Weight of urea taken, w2= 50 g

Molecular weight of water, M1= 18 g mol - 1

Molecular weight of urea, M2= 60 g mol - 1

Now, we have to calculate vapour pressure of water in the solution. We take vapour pressure as p1.

Now, from Raoult's law, we have:

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

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