The relative lowering of vapour pressure is_____________________?
Correct answer: D. both B & C
- A. equal to the mole fraction of solvent
- B. equal to the mole fraction of solute
- C. directly proportional to the mole fraction of solute
- D. both B & C
Explanation
For a solution containing a non-volatile solute, Raoult's law gives (P° − P) / P° = Xsolute, so relative lowering of vapour pressure equals the mole fraction of solute and is directly proportional to it. Students may choose the mole fraction of solvent because vapour pressure depends on solvent, but the relative lowering is expressed in terms of solute mole fraction.
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About Evaporation, Boiling Point and Vapour Pressure
Evaporation is the escape of higher-energy molecules from a liquid surface, while vapour pressure is the pressure exerted by vapour in equilibrium with that liquid. The topic distinguishes evaporation from boiling and relates both to temperature, intermolecular forces, surface area, atmospheric pressure and the boiling point.
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