In thermodynamics, the cryoscopic constant, Kf, allows one to relate molality to freezing point depression. It is the ratio of the latter to the former:
where i is the van 't Hoff factor, the number of particles the solute splits into or forms when dissolved.
Through the procedure called cryoscopy, a known constant can be used to calculate an unknown molar mass. The term "cryoscopy" comes from the Greek language and means "freezing measurement." This is related to ebullioscopy, which determines the same value from the ebullioscopic constant (of boiling point elevation).
The Kf for water is 1.853 K·kg/mol. Aylward, Gordon; Findlay, Tristan (2002), SI Chemical Data 5th ed. (5 ed.), Sweden: John Wiley & Sons, pp. 202, ISBN 0470800445
* Freezing-point depression