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Colligative Properties Depend on the concentration of solute particles, but not on chemical identity. In the case of a solute that does not ionize, concentration of solute particles has the same meaning as solute concentration.

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Some Colligative Properties Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure

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Why Vapor Pressure Lowering? Results when a non-volatile solute is dissolved in a volatile solvent. Only some of the surface molecules have the ability to vaporize, compared to all of the surface molecules in the case of a pure solvent. As a result, the vapor pressure of the solution is less than that of a pure solvent.

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Molecular-Level Explanation for Vapor Pressure Lowering (image 1 of 3)

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Molecular-Level Explanation for Vapor Pressure Lowering (image 2 of 3)

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Molecular-Level Explanation for Vapor Pressure Lowering (image 3 of 3)

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An Interesting Illustration of the Difference in Vapor Pressure Between Solvent and Solution

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Raoults Law P = X. P o Obeyed exactly, only for ideal solutions For non-ideal solutions, Raoults law is more closely followed for dilute solutions

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Raoults Law Applied to Both Solvent and Solute (when both are volatile) P = X. P o P = P + P = X. P o + X. P o = (1-X). P o + X. P o

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Vapor Pressure Lowering Changes the Boiling and Freezing Points

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Equations for Boiling Point Elevation and Freezing Point Depression T b = T b T o b and T b = K b. c m T f = T f T o f and T f = -K f. c m

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