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A solution is prepared by dissolving a non-volatile solute in a solvent. If the vapour pressure of the resulting solution is 75 % of the vapour pressure of the pure solvent, what is the ratio of the number of moles of solute to the number of moles of solvent in the solution?

Options

  1. A1:4
  2. B3:1
  3. C3:4
  4. D1:3

Correct answer

D. 1:3

Step-by-step solution

Let the vapour pressure of the pure solvent be P^ . The vapour pressure of the solution is P_ s = 0.75 P^ . According to Raoult's law, the relative lowering of vapour pressure is equal to the mole fraction of the solute ( X_ solute ): X_ solute = P^ - P_ s P^ = P^ - 0.75 P^ P^ = 0.25 = 1 4 By definition, X_ solute = n_ solute n_ solute + n_ solvent n_ solute n_ solute + n_ solvent = 1 4 4 n_ solute = n_ solute + n_ solvent 3 n_ solute = n_ solvent n_ solute n_ solvent = 1 3 The ratio of the number of moles of solut

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