15 g of urea is dissolved in 18 g of water. If the relative lowering in vapour pressure of the solution is P, find the value of 10*P.

Correct answer: 2

Solution

By Raoult's law the relative lowering of vapour pressure equals the mole fraction of the solute: (P* - P_solution)/P* = x_urea. Molar mass of urea (NH2CONH2) = 2(14) + 4(1) + 12 + 16 = 60 g/mol. Moles of urea = 15/60 = 0.25. Moles of water = 18/18 = 1. x_urea = 0.25/(0.25+1) = 0.25/1.25 = 1/5 = 0.2. Therefore P = 0.2 and 10P = 2.

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