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Question
Answer the following.
Why vapour pressure of a solvent is lowered by dissolving a nonvolatile solute into it?
Solution
i. Vapour pressure of a liquid depends on the ease with which the molecules escape from the surface of the liquid.
ii. When a nonvolatile solute is dissolved in a solvent, some of the surface molecules of the solvent are replaced by nonvolatile solute molecules. These solute molecules do not contribute to vapour above the solution.
iii. 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 the pure solvent.
iv. 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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