Cryoscopic constant
Encyclopedia
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.
where i is the van 't Hoff factor
Van 't Hoff factor
The van 't Hoff factor i is a measure of the effect of a solute upon colligative properties, such as vapor pressure, osmotic pressure and freezing point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved, and the...
, 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
Molar mass
Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
. The term "cryoscopy" comes from the Greek language
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
and means "freezing measurement." This is related to ebullioscopy, which determines the same value from the ebullioscopic constant
Ebullioscopic constant
In thermodynamics, the ebullioscopic constant, Kb, allows one to relate molality to boiling point elevation. It is the ratio of the latter to the former:\Delta T = i\cdot K_b \cdot mi is the Vant Hoff factor...
(of boiling point elevation).
The Kf for water is 1.853 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
·kg/mol.
See also
- Freezing-point depressionFreezing-point depressionFreezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...
- List of boiling and freezing information of solvents