Chemistry Essay

774 WordsSep 18, 20134 Pages
Colligative Properties Key words: Vapor pressure lowering | Boiling point elevation | Freezing point depression | Osmotic pressure | Conceptual questions | The colligative properties of a solution depend on the relative numbers (concentration) of solute and solvent particles, they do not depend on the nature of the particles. Colligative properties change in proportion to the concentration of the solute particles. We distinguish between four colligative properties: * vapor pressure lowering * freezing point depression * boiling point elevation * osmotic pressure. All of the colligative properties fit the relationship property = solute concentration x constant Property | Symbol | Solute Concentration | Proportionality Constant | Vapor pressure | P | mole fraction | Po (vapor pressure of pure solvent) | Boiling Point | Tb | molal | Kb (bpoilng point constant) | Freezing Point | Tf | molal | Kf (freezing point constant) | Osmotic Pressure |  | molar | RT | The determination of colligative properties allows us to * determine the concentration of a solution * calculate molar masses of solutes Vapor Pressure Lowering: The vapor pressure of a solvent in a solution is always lower than the vapor pressure of the pure solvent. The vapor pressure lowering is directly proportional to the molefraction of the solute. Psolvent = Xsolvent Posolvent or P = Xsolute Posolvent where Psolvent is the vapor pressure of the solvent on the solution, Xsolvent and Xsolute is the mole fraction of the solvent or solute, respectively. (Xsolvent + Xsolute = 1). Posolvent is the vapor pressure of the pure solvent, and P is the lowering in vapor pressure. Exercise: At 25oC the vapor pressure of pure benzene is 93.9 torr. When a non-volatile solvent is dissolved in benzene, the vapor pressure of benzene is lowered to 91.5

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