Freezing Point Depression (Molality) Calculator
Calculate how far a dissolved solute lowers a solvent’s freezing point from its molality, cryoscopic constant and van’t Hoff factor.
How it works
ΔTf = i × Kf × molality · new freezing point = normal freezing point − ΔTf
Freezing point depression is a colligative property: dissolving any non-volatile solute lowers the temperature at which a solvent freezes, and the size of the drop depends only on how many solute particles are present, not what they are. The van’t Hoff factor i counts how many particles each formula unit releases — 1 for sugar, roughly 2 for NaCl — while Kf is a property of the solvent.
Worked example
Dissolving a non-electrolyte to 0.5 mol/kg in water (Kf = 1.86, i = 1): ΔTf = 1 × 1.86 × 0.5 = 0.93 °C. The freezing point falls from 0 °C to −0.93 °C.
Frequently asked questions
What is the van’t Hoff factor?
It is the number of particles a solute splits into when dissolved. Non-electrolytes like sugar give i ≈ 1; NaCl dissociates into Na⁺ and Cl⁻ so i ≈ 2; CaCl₂ gives i ≈ 3. More particles mean a larger freezing-point drop.
Why is molality used instead of molarity?
Molality (moles per kilogram of solvent) does not change with temperature, whereas molarity (per litre of solution) does because volume expands or contracts. Colligative-property equations therefore use molality.
Is this why salt melts ice on roads?
Yes. Salt dissolves in the thin water layer and depresses its freezing point, so ice melts at temperatures below 0 °C where it otherwise would not.
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This calculator is for educational purposes. Double-check important results before acting on them.