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.

Q. The vapour pressure of water at

20^{o} C

is 17.53 mm. Calculate the vapour pressure of a solution at

20^{o} C

containing 6 g of urea in 100 g of water (molecular mass of urea = 60).

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18.2 g of urea is dissolved in 100g of water at 50oC. The lowering of vapour pressure produced is 5 mm Hg. Calculate the molecular mass of urea. The vapour pressure of water at 50oC is 92 mm Hg.

The correct option is C 57.05We have,PoB−PtP0B=xA592=nAnA+nB = xx+5.55 we get x = 0.316 moles molecular weight = weightmoles molecular weight of urea = 57.05

18.2 g of urea is dissolved in 100g of water at $50^{o} C$ . The lowering of vapour pressure produced is 5 mm Hg. Calculate the molecular mass of urea. The vapour pressure of water at $50^{o} C$ is 92 mm Hg.

The correct option is C 57.05
We have,
$\frac{P_{B}^{o} - P_{t}}{P_{B}^{0}} = x_{A}$

$\frac{5}{92} = \frac{n_{A}}{n_{A} + n_{B}}$ = $\frac{x}{x + 5.55}$
we get x = 0.316 moles
molecular weight = $\frac{w e i g h t}{m o l e s}$
molecular weight of urea = 57.05