23°C the vapour pressure of pure water is 23.76 mm of Hg and that of an aqueous dilute solution of urea is 22.98 mm of Hg . calculate the molarity of the solution .

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Solution

we know that,
total vapour pressure = pure vapour pressure X mole fraction
22.98 = 23.76 X Xw
therefore,
Xw = 22.98/23.76
=Nw/Nw+ Na
so,
22.98/23.76--22.98
====>Nw/Na
therefore,
Na/Nw = 23.76--22.98/22.98

NOW,
molality = Na/mass of water in kg
=Na/Nw X 18X10^-3
so molality = 23.76--22.98/22.98 X 18X10^-3
so the final answer = 0.001X 10^-3

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