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 ).

See also

• Cold mixture

Individual evidence

1. ^ Gordon Aylward, Tristan Findlay: SI Chemical Data , Sweden, John Wiley & Sons, 2002, 5th edition, ISBN 0-470-80044-5 .
2. ↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 4: M-Pk. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1985, ISBN 3-440-04514-5 , pp. 2650-2651.