Cryoscopic Constant
Definition and meaning of Cryoscopic Constant in chemistry.
The cryoscopic constant (Kf) is the proportionality constant that relates the freezing point depression of a solvent to the molal concentration of dissolved solute particles. It is also called the molal freezing point depression constant.
In more detail
Kf appears in the colligative property equation ΔTf = Kf × m × i, where m is molality and i is the van't Hoff factor accounting for solute dissociation. Its value depends only on the identity of the solvent (through the solvent's molar mass, enthalpy of fusion, and normal freezing point), not on which solute is dissolved. This solvent-specific, solute-independent nature makes Kf useful in cryoscopy, a classical technique for determining the molar mass of an unknown nonvolatile solute by measuring how much it lowers a solvent's freezing point.
Key facts
| Field | Physical Chemistry |
|---|---|
| Symbol | Kf |
| Units | °C·kg/mol (K·kg/mol) |
| Kf of water | 1.86 °C·kg/mol |
Water has Kf = 1.86 °C·kg/mol. Dissolving 1 mole of a nonelectrolyte solute (i = 1) in 1 kg of water lowers the freezing point by 1.86°C, giving a new freezing point of −1.86°C instead of 0°C.
Frequently asked questions
Does the cryoscopic constant depend on the solute?
No. Kf is a fixed property of the solvent alone; the identity of the solute does not change its value, only the amount of freezing point depression observed.
How does Kf differ from the ebullioscopic constant?
Kf governs freezing point depression while the ebullioscopic constant (Kb) governs boiling point elevation for the same solvent; the two constants generally have different numerical values because they depend on different solvent properties (enthalpy of fusion versus enthalpy of vaporization).