Solutions II

# Solutions II - COLLIGATIVE PROPERTIES Colligative...

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COLLIGATIVE PROPERTIES

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Colligative Properties of Solutions Colligative properties are properties of solutions that depend solely on the number of particles dissolved in the solution. Colligative properties do not depend on the kinds of particles dissolved. Colligative properties are a physical property of solutions.
Colligative Properties of Solutions There are four common types of colligative properties: 1. Vapor pressure lowering 2. Freezing point depression 3. Boiling point elevation 4. Osmotic pressure Vapor pressure lowering is the key to all four of the colligative properties.

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Lowering of Vapor Pressure and Raoult’s Law Addition of a nonvolatile solute to a solution lowers the vapor pressure of the solution. The effect is simply due to fewer solvent molecules at the solution’s surface. The solute molecules occupy some of the spaces that would normally be occupied by solvent. Raoult’s Law models this effect in ideal solutions.
Lowering of Vapor Pressure and Raoult’s Law

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Lowering of Vapor Pressure and Raoult’s Law Derivation of Raoult’s Law. P P where P vapor pressure of solvent P vapor pressure of pure solvent mole fraction of solvent solvent solvent solvent 0 solvent solvent 0 solvent X in solution X in solution
Lowering of vapor pressure, P solvent , is defined as: 0 solvent solvent 0 solvent solvent 0 solvent solvent 0 solvent solvent )P 1 ( ) P )( ( - P P P P X X Lowering of Vapor Pressure and Raoult’s Law

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Remember that the sum of the mole fractions must equal 1. Thus X solvent + X solute = 1, which we can substitute into our expression. Law s Raoult' is which P P - 1 0 solvent solute solvent solvent solute X X X Lowering of Vapor Pressure and Raoult’s Law
Fractional Distillation Distillation is a technique used to separate solutions that have two or more volatile components with differing boiling points. A simple distillation has a single distilling column. Simple distillations give reasonable separations. A fractional distillation gives increased separations because of the increased surface area. Commonly, glass beads or steel wool are inserted into the distilling column.

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Fractional Distillation
Boiling Point Elevation Addition of a nonvolatile solute to a solution raises the boiling point of the solution above that of the pure solvent. This effect is because the solution’s vapor pressure is lowered as described by Raoult’s law. The solution’s temperature must be raised to make the solution’s vapor pressure equal to the atmospheric pressure. The amount that the temperature is elevated is determined by the number of moles of solute dissolved in the solution.

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Boiling Point Elevation Boiling point elevation relationship is: solvent for the constant elevation point boiling molal K solute of ion concentrat molal elevation point boiling T : where K T b b b b m m
Boiling Point Elevation

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