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.

Dear Aryan,

According to the Raoult's Law :

                              P_sol = x P_o

where P_o = vapour pressure of the pure solution

               x =  mole fraction of the water

          P_sol =  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.