Ebullioscopic Constant of Water
| Value | 0.512 K·kg/mol |
| Status | Measured: ± 0.001 K·kg/mol (0.002 relative) |
| Source | CRC Handbook |
| Categories | Chemistry |
| kelvin kilogram per kilomole | 512 K·kg/kmol |
| kelvin kilogram per mole | 0.512 K·kg/mol |
| Celsius kilogram per mole | 0.512 °C·kg/mol |
| kelvin per molal | 0.512 K/molal |
| Celsius degree per molal | 0.512 °C/molal |
Learning zone
The mirror of freezing-point depression: dissolved particles lower the vapour pressure of the solvent, so the solution must be heated further before its vapour pressure reaches atmospheric. ΔTb = i·Kb·m, and at 0.512 K·kg/mol the effect is nearly four times weaker than the freezing-point effect. This is why the kitchen claim that salting pasta water makes it boil hotter is technically true and practically irrelevant: a tablespoon of salt in four litres raises the boiling point by about 0.1 °C.
The constant follows from the solvent's own properties — Kb = RTb²M/ΔHvap — so water's high enthalpy of vaporisation is precisely what makes its value small. Solvents with weak intermolecular forces do far better: camphor's cryoscopic constant is 40 K·kg/mol, which made it the classic solvent for molar-mass determination by the Rast method. In industry the effect is a real design load in evaporators, where a concentrated liquor boils meaningfully above the temperature of pure water at the same pressure.