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When a non-volatile solute is added to a solvent what is the difference in vapor pressure expressed as a faction of original vapor pressure equal to?(a) Mole fraction of solute in vapor phase(b) Mole fraction of solvent in vapor phase(c) Mole fraction of solute in liquid phase(d) Mole fraction of solvent in liquid phaseThe question was asked during an interview.My question is from Colligative Properties and Determination of Molar Mass in chapter Solutions of Chemistry – Class 12

Answer»

Right ANSWER is (c) Mole fraction of solute in liquid phase

To ELABORATE: Relative lowering of vapor pressure is the difference in vapor pressure expressed as a fraction of original vapor pressure. Using RAOULT’s law:

ΔP1 = X2 x P1

Therefore, ΔP1/P1 = X2 which implies the relative lowering is EQUAL to mole fraction of solute in liquid phase. Since the solute is non-volatile, it cannot be present in vapor phase.



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