What Is a Freezing Point Depression Calculator?
Freezing point depression is a colligative property: dissolving any solute in a solvent lowers the solvent's freezing point, and the amount of the drop depends only on how many particles are dissolved — not their chemical identity. This calculator computes the molality of your solution from solute mass, molar mass and solvent mass, then applies ΔTf = i·Kf·m to find the new freezing point.
It's the same math behind road salt melting ice and antifreeze (ethylene glycol) protecting a car's radiator from freezing.
How to Read Your Results
New Freezing Point
The temperature at which the solution will start to freeze, after accounting for the dissolved solute.
Molality
Moles of solute per kilogram of solvent — the concentration unit used for colligative property calculations because it doesn't change with temperature (unlike molarity, which is volume-based).
Freezing Point Depression (ΔTf)
How many degrees the freezing point drops below the pure solvent's freezing point.
Moles of Solute
The total moles of solute dissolved, calculated as solute mass ÷ solute molar mass.
Real-World Example
Dissolving 10 g of table salt (NaCl, molar mass 58.44 g/mol, i = 2) in 1 kg of water (Kf = 1.86 °C·kg/mol, pure freezing point 0 °C):
| Quantity | Value |
|---|---|
| Moles of NaCl | 10 ÷ 58.44 ≈ 0.1711 mol |
| Molality | 0.1711 mol/kg |
| ΔTf | 2 × 1.86 × 0.1711 ≈ 0.64 °C |
| New freezing point | 0 − 0.64 ≈ −0.64 °C |
The salted water needs to reach about −0.64 °C before it starts to freeze, instead of 0 °C for pure water — the same principle behind salting icy roads.
Tips for Accurate Results
- Use the correct van't Hoff factor for ionic compounds — they dissociate into multiple particles, multiplying the effect.
- Common Kf values: water 1.86, benzene 5.12, camphor 40.0 °C·kg/mol.
- This is an ideal-solution approximation — real solutions can deviate at high concentrations due to ion pairing.
- Use freezing point depression experimentally to determine an unknown solute's molar mass if you know the mass and ΔTf.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team