Colligative Properties Calculator
Calculate boiling point elevation, freezing point depression, osmotic pressure, vapor pressure lowering and molality, with the van't Hoff factor.
What this calculator does
Enter the moles of solute, the mass of solvent and the van't Hoff factor to get molality and the change in boiling point, freezing point, osmotic pressure or vapor pressure.
When you need it
Colligative properties explain why salt melts ice, why antifreeze works and how the molar mass of an unknown solute can be measured from a freezing point change.
Formulas
- ΔTb = i · Kb · m
- ΔTf = i · Kf · m
- π = i · M · R · T
- ΔP = χ(solute) · P°
How to use it
- Enter the moles of solute and the mass of the solvent to get molality.
- Enter the van't Hoff factor i (1 for sugar, about 2 for NaCl).
- Pick the property you need and read the result.
Worked example
What is the freezing point of a solution of 0.250 mol sucrose in 500 g of water? (Kf = 1.86 °C·kg/mol)
- m = 0.250 mol ÷ 0.500 kg = 0.500 mol/kg
- ΔTf = 1 × 1.86 × 0.500 = 0.93 °C
- Freezing point = 0.00 − 0.93 = −0.93 °C
Answer
The solution freezes at about −0.93 °C.
Common mistakes
- Using the solvent's mass in grams for molality instead of kilograms.
- Forgetting i for salts, which split into ions.
- Subtracting the freezing point change in the wrong direction.
Related tools
Frequently asked questions
What are colligative properties?
Properties of a solution that depend on the number of dissolved particles, not on what they are: boiling point elevation, freezing point depression, vapor pressure lowering and osmotic pressure.
What is the van't Hoff factor?
The number of particles a solute forms in solution. Sugar stays whole (i = 1), while NaCl gives two ions (i ≈ 2).
Why does salt melt ice?
Dissolved salt lowers the freezing point of water, so ice melts at a temperature below 0 °C.