Cryoscopic constant

In thermodynamics, the cryoscopic constant, K_f, relates molality to freezing point depression (which is a colligative property). It is the ratio of the latter to the former:

$\triangle T_{f}=i\cdot K_{f}\cdot b$

i is the van 't Hoff factor, the number of particles the solute splits into or forms when dissolved.b is the molality of the solution.

Through cryoscopy, a known constant can be used to calculate an unknown molar mass. The term "cryoscopy" comes from Greek and means "freezing measurement." Freezing point depression is a colligative property, so $\Delta T$ depends only on the number of solute particles dissolved, not the nature of those particles. Cryoscopy is related to ebullioscopy, which determines the same value from the ebullioscopic constant (of boiling point elevation).

The K_f for water is 1.853 K·kg/mol.

See also

• List of boiling and freezing information of solvents