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##### Vapor Pressure

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**Vapor Pressure**William Henry Francois Marie Raoult**Henry’s Law**The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution Applies most accurately for dilute solutions of gases that do not dissociate or react with the solvent Yes CO2, N2, O2 No HCl, HI**Raoult’s Law**The presence of a nonvolatile solute lowers the vapor pressure of the solvent. Psolution = Observed Vapor pressure of the solution solvent= Mole fraction of the solvent P0solvent = Vapor pressure of the pure solvent**Liquid-liquid solutions in which both components are**volatile Modified Raoult's Law: P0 is the vapor pressure of the pure solvent PAand PB are the partial pressures**Raoult’s Law – Ideal Solution**A solution that obeys Raoult’s Law is called an ideal solution**Negative Deviations from Raoult’s Law**Strong solute-solvent interaction results in a vapor pressure lower than predicted Exothermic mixing = Negative deviation**Positive Deviations from Raoult’s Law**Weak solute-solvent interaction results in a vapor pressure higher than predicted Endothermic mixing = Positive deviation