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Question
Give two points to explain why vapour pressure of solvent is lowered by dissolving nonvolatile solute into it.
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Solution
- Vapour pressure of a liquid depends on the ease with which the molecules escape from the surface of liquid. When nonvolatile solute is dissolved in a solvent, some of the surface molecules of solvent are replaced by nonvolatile solute molecules. These solute molecules do not contribute to vapour above the solution.
- Thus, the number of solvent molecules available for vaporization per unit surface area of a solution is less than the number at the surface of pure solvent. As a result the solvent molecules at the surface of solution vaporize at a slower rate than pure solvent. This results in lowering of vapour pressure.
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