Boiling-Point Elevation and Freezing-Point Depression Constants
Molal boiling-point-elevation (Kb) and freezing-point-depression (Kf) constants for common solvents. Use ΔTb = i·Kb·m and ΔTf = i·Kf·m, where m is the molality of the solution and i is the van 't Hoff factor.
| Solvent | Formula | Kb (bp elevation) | Kf (fp depression) |
|---|---|---|---|
| Water | H2O | 0.512 °C·kg/mol | 1.86 °C·kg/mol |
| Acetic acid | CH3COOH | 3.07 °C·kg/mol | 3.9 °C·kg/mol |
| Benzene | C6H6 | 2.53 °C·kg/mol | 5.12 °C·kg/mol |
| Chloroform | CHCl3 | 3.63 °C·kg/mol | 4.68 °C·kg/mol |
| Nitrobenzene | C6H5NO2 | 5.24 °C·kg/mol | 8.1 °C·kg/mol |
| Ethanol | C2H5OH | 1.20 °C·kg/mol | — |
| Carbon disulfide | CS2 | 2.34 °C·kg/mol | — |
| Camphor | C10H16O | — | 37.7 °C·kg/mol |
Important Notes
- ΔTb = i·Kb·m raises the boiling point; ΔTf = i·Kf·m lowers the freezing point. Both use MOLALITY (mol solute per kg solvent), not molarity.
- The van 't Hoff factor i counts dissolved particles: i = 1 for nonelectrolytes, i ≈ the ion count for strong electrolytes (e.g. i ≈ 2 for NaCl, i ≈ 3 for CaCl₂).
- A dash means that constant is not used in this course for that solvent.
- Camphor's very large Kf made it the classic solvent for molar-mass determination by freezing-point depression (the Rast method).
- These are the course-canonical values: every ChemWhiz colligative problem uses the constants in this table.
Source: ChemWhiz course-canonical colligative constants (from the Topic 19 course rules)