Colligative Properties- is characteristic of the solvent and depends on the concentration but not the nature of the solute.(boiling point elevation and freezing point dpression are both colligative properties.
Freezing Point- The temperature at which a solid turns to a liquid.
Freezing point depression- occurs when a solute is dissolved in a solvent. The freezing point of the solution will be lower than that of the original pure solute.
The amount that the freezing point is lowered depends on two factors.
1.The concentration of the solute. The more solute the more the freezing point will be lowered. Thus a 2 molal solution of sugar and water will have a lower boiling point than a 1 molal solution of water and sugar.
2.The nature of the Solvent. Two solution of the same concentration of solute will freeze at different temperatures if they are composed of different solvents.
The relationship between lowering of freezing point and the molal concentration of a solution can be expressed as
DTf=Kfm
DT= depression of freezing point
K= molal freezing point constant
m= molal concentration of the solution
What is the freezing point of a solution of 15.0 g of Na Cl in 250 g of water? The molal freezing point constant, Kf, for water is 1.86 oC kg / mol.
DTf=Kfm
The concentration of the solution is 1.03 m in Na Cl. Therefore, the change in the freezing point of the water is 1.97 oC. The freezing point of the solution is, therefore, 1.97oC.
Boiling Point- The point at which the Vapor pressure of the solution is equal to the pressure of the atmosphere.
Boiling Point Elevation- A solution boils at a higher temperature than that of the pure solvent.
DTb=Kbm
What is the boiling point of a 1.2 m aqueous solution of a nonelectrolyte? The Kb of water is 0.51 oC kg / mol.
(A) 100.6 oC (B) 99.4 oC (C) 0.612 oC (D) None of the above
answer C.
To test yourself on this material use
do problems 20, 21, 23 on page 460
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