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Raoult's Law and Vapor Pressure Lowering

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State

Raoult's law states a solution's vapor pressure equals the pure solvent's vapor pressure times the solvent's mole fraction, so a nonvolatile solute lowers it.

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Dissolving glycerin, a nonvolatile solute, in ethanol lowers the solution's vapor pressure below that of pure ethanol.

Watch for

This form covers a nonvolatile solute — a volatile second component would add its own partial pressure instead.