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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. - CBSE (Science) Class 12 - Chemistry

ConceptVapour Pressure of Liquid Solutions Vapour Pressure of Liquid- Liquid Solutions

#### Question

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.

#### Solution

It is given that vapour pressure of water, p_1^0= 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:

(p_1^0 - p_1)/p_1^0 = n_2/(n_1+n_2)

=>(p_1^0-p_1)/p_1^0 = (w_2/M_2)/(w_1/M_1+w_2/M_2)

=>(23.8-p_1) /23.8 = (50/60)/(850/18 + 50/60

=>(23.8-p_1)/23.8 = 0.83/(47.22+0.83)

=>(23.8-p_1)/23.8= 0.0173

=> p_1 = 23.4 "mm of Hg"

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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Solution 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. Concept: Vapour Pressure of Liquid Solutions - Vapour Pressure of Liquid- Liquid Solutions.
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