Question - 2.9 - Vapour pressure of pure water at 298 K is 23.8 mm Hg. 50 g of urea (NH2 CONH2 ) is dissolved in 850 g of water. Calculate the vapour pressure of water for this solution and its relative lowering.
Weight of Urea (WB ) = 50 g
Molar mass (MB ) of Urea (NH2 CONH 2) = 14 + 1 x 2 + 12 +16 +14 + 1 x 2 = 60 g mol – 1
Weight of water (W A) = 850 g
Molar mass of water (M A) = 18 g mol – 1
Vapour pressure of water (P A o) = 23.8 mm Hg
Vapour pressure of water in the given solution P A = ?
Now, number of moles of urea
Now, number of moles of water
Now, Mole fraction of urea
Now, we know that,
Thus, vapour pressure of water in this solution is 23.40 mm of Hg and its relative lowering is 0.017