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The vapour pressure of pure water is 23.76 mm of Hg and that of aqueous dilute soluton of urea 22.98 mm of Hg.Find the molality of solution.

7 years ago


Answers : (1)


Dear Aryan,

According to the Raoult's Law :

                              Psol = x Po

where Po = vapour pressure of the pure solution

               x =  mole fraction of the water

          Psol =  vapour pressure of the solution.

 Putting the given values in the equation:

                                   22.98 = x(23.76)

                                           x = 0.96717

                                              = 0.97

Mole fraction of solute = 0.03

Therefore molality = ( moles of solute) / weight of solvent (in kg)

                                 = (mole fraction of solute)1000 /( mole fraction of solvent * 18)

                                 = (0.03 * 1000) / (0.97 * 18)

                                 = 1.7182 m


Note ; The vapour pressure of Urea will be zero.


Thank you.

7 years ago

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