Vapour pressure lowering : When a liquid in a closed container is in equilibrium with its vapours, the pressure exerted by the vapour on the liquid is its vapour pressure.
i. Experiments have shown that when a nonvolatile, nonionizable solid is dissolved in a liquid solvent, the vapour pressure of the solution is lower than that of pure solvent. In other words the vapour pressure of a solvent is lowered by dissolving a nonvoltile solute into it. When the solute is nonvolatile it does not contribute to the vapour pressure above the solution. Therefore, the vapour pressure of solution is equal to the vapour pressure of solvent above the solution.
ii. If P is the vapour pressure of pure solvent and P, is the vapour pressure of solvent above the solution, then
P1< P⁰1
The vapour pressure lowering is
ΔP = P⁰1 - P1
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