K·kg/mol
kelvin kilogram per mole
Molal constantexact by definition
The kelvin kilogram per mole is the unit of the molal freezing-point depression and boiling-point elevation constants, Kf and Kb. It is the coherent SI form and its factor of 1 is exact. For water Kf is 1.86 K·kg/mol and Kb is 0.512 K·kg/mol, both measured properties of the solvent rather than defined values.
Watch out: The constant belongs to the solvent, not the solute. Benzene has Kf = 5.12 and camphor an enormous 40 or so, which is why camphor was the classic solvent for determining molar masses by freezing-point depression: a large constant makes a small amount of solute produce a measurable temperature shift.
| 1 K·kg/mol | 1 K·kg/mol |
Solvers that use molal constant