Raoult's Law Calculator (Boiling Point Elevation & Freezing Point Depression)
Calculate boiling point elevation and freezing point depression from a solute's molality and van't Hoff factor using Raoult's law. Supports six solvent constants (Kb/Kf) including water and benzene, and accounts for electrolyte dissociation.
Molal boiling/freezing point constants by solvent
Standard reference values widely cited in textbooks. Use them to check your own hand calculations.
| Solvent | Kb (°C·kg/mol) | Kf (°C·kg/mol) | Normal bp (°C) | Normal fp (°C) |
|---|---|---|---|---|
| Water | 0.512 | 1.86 | 100.0 | 0.0 |
| Benzene | 2.53 | 5.12 | 80.1 | 5.5 |
| Acetic acid | 3.07 | 3.9 | 118.1 | 16.6 |
| Cyclohexane | 2.79 | 20.2 | 80.7 | 6.5 |
| Naphthalene | 5.8 | 6.9 | 218.0 | 80.2 |
| Camphor | 5.95 | 37.7 | 204.0 | 178.4 |
Tips
- Electrolytes such as NaCl or CaCl2 dissociate into multiple ions in water, so setting the van't Hoff factor i above 1 brings the result closer to real measurements.
- In the mass-based mode, just enter the mass of solute [g], its molar mass [g/mol], and the mass of solvent [kg] to have the molality calculated automatically.
- Both boiling point elevation and freezing point depression are colligative properties: they depend only on the number of solute particles, not on what the solute actually is.
- For a given solute amount, freezing point depression is often larger than boiling point elevation for common solvents, which is one reason calcium chloride is used as a road de-icer.
- Use the i preset buttons (non-electrolyte / NaCl-like / CaCl2-like) to quickly try out common dissociation patterns.
Frequently Asked Questions
Side Note — De-icers and the chemistry of colligative properties
The white granules scattered on winter roads are often calcium chloride (CaCl2) or sodium chloride (NaCl). These substances melt ice and snow not by warming them, but by lowering the freezing point of water itself once dissolved in it. This phenomenon, freezing point depression, is a classic example of the colligative properties that follow from Raoult's law.
The word "colligative" captures the idea that only the number of particles matters, not their identity. At the same molality, CaCl2 dissociates in water into Ca2+ and two Cl- ions — three particles in total — producing roughly three times the freezing point depression of a non-dissociating non-electrolyte like sucrose. That's one reason CaCl2 can act as a more powerful de-icer than NaCl in smaller quantities (though its corrosive effect on concrete is a separate consideration).
The same principle applies in the opposite direction to boiling point elevation. Adding salt to pasta water does raise its boiling point slightly, but at the amounts used in home cooking the rise is well under 1°C — nowhere near enough to make the water boil noticeably faster. The main reason cooks add salt is really to season the pasta; the boiling point elevation is just a minor chemical side effect.