# Cryoscopic constant

Absolute Beckmann thermometer

The cryoscopic constant (from cryoscopy , see below) is a measured variable from physical chemistry . It indicates the temperature (in Kelvin ) by which the melting point of a solution is reduced compared to that of the pure solvent (melting point or freezing point lowering ) when 1  mol of a non- volatile substance is dissolved in 1 kg of the pure solvent. ${\ displaystyle E_ {n}}$

The cryoscopic constant can be calculated using the following formula:

${\ displaystyle E_ {n} = {\ frac {M_ {n} \ cdot R \ cdot T ^ {2}} {\ Delta H _ {\ text {m, fus}}}}}$
• ${\ displaystyle M_ {n}}$: molar mass of the solvent
• ${\ displaystyle R}$: universal gas constant
• ${\ displaystyle T}$: Melting temperature at a given pressure
• ${\ displaystyle \ Delta H _ {\ text {m, fus}}}$: molar enthalpy of fusion

## Some values

solvent Freezing point in ° C Cryoscopic constant
in K kg / mol
water 0 −1.853
naphthalene 80.2 −6.80
chloroform −63.5 −4.68
benzene 5.5 −5.12
Camphor 179 −39.7
Ethanol −114.6 −1.99
Cyclohexane 6.4 −20.2
Carbon tetrachloride −23 −30

## application

The cryoscopic constant can be used to determine the molar mass of a soluble substance from the depression of the freezing point by cryoscopy (for example according to Beckmann using a Beckmann thermometer ).