Freezing-point depression
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Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent. It is directly proportional to the molal concentration of the solution according to the equation:
freezing-point depression = i · Kf · molality
- molality is in units of mol/kg
- Kf, the freezing-point depression constant is a colligative property.
- Kf for water is 1.86 K·kg/mol which means that per mole of solute dissolved in a kilogram of water the freezing point depression is 1.86 kelvins.
- i is the i factor or the van 't Hoff i factor see van 't Hoff
- i is the factor that takes into account the presence of ions in a solution, it indicates the number of particles formed.
Examples:
- i = 1 for sugar in water
- i = 2 for NaCl in water
- i = 3 for CaCl2 in water
- i = 2 for HCl in water (complete dissociation)
- i = 1 for HCl in benzene (no dissociation)