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Colligative Properties

Colligative properties are determined by the number of particles present in a system. This article explains freezing point depression and boiling point elevation, and provides formulas for calculating these changes. Examples are also given to illustrate the application of colligative properties.

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Colligative Properties

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  1. Colligative Properties

  2. What is a colligative property? • A property of a substance or system that is determined by the number of particles present in the system but independent of the particles themselves. • In other words, it doesn’t matter what it is but how much there is. • Examples: • Freezing point depression – lowering of the freezing point of a solution (salt on roads in winter) • Boiling point elevation – raising of the boiling point of a solution.

  3. The formulas • The change in boiling or freezing point can be calculated using vary similar formulas. • Tf = kfmN, where • Tf is the change in the freezing point • kf is the freezing point constant for the solvent • mis the molality • Nis number of particles in the solution • For nonelctrolytes, N = 1 • For electrolytes (acids and ionic compounds), N = number of dissociated particles. • The other formula is Tb = kbmN and this is for boiling point elevation

  4. Values for some common solvents

  5. Examples • What is the freezing point of a solution of 210.0 g of glycerol, C3H8O3, dissolved in 350 g of water, if the normal freezing point of water is 0.00 C and kf is -1.86 C/m? • What is the boiling point of a solution containing 34.3 g of the ionic compound magnesium nitrate in 0.107 kg of water, if the normal boiling point of water is 100.0 C and kb is 0.51 C/m?

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