Freezing Point Depression Calculator

Formula

How this is calculated

Freezing point depression is a colligative property: adding a solute to a solvent disrupts the formation of the crystal lattice, so the solution must be cooled further before it freezes. The magnitude of the effect depends only on the number of dissolved particles in the solvent, not on their chemical identity.

The formula ΔTf = Kf × m × i links three quantities. Kf is the cryoscopic constant of the solvent — a fixed experimental value in °C·kg/mol that reflects how strongly the solvent's freezing is affected per mole of solute particles (1.86 for water, 5.12 for benzene, 20.0 for cyclohexane). m is the molality of the solution in mol/kg, a temperature-independent concentration measure. i is the van't Hoff factor — the effective number of particles each formula unit produces in solution: 1 for non-electrolytes such as sugar or ethanol, 2 for NaCl (Na⁺ + Cl⁻), 3 for CaCl₂ (Ca²⁺ + 2 Cl⁻). The new freezing point of the solution is Tf(pure) − ΔTf.

This formula gives the ideal-dilute-solution result. At high molalities (above roughly 0.5 mol/kg for water) real solutions deviate from ideal behaviour because ion–ion interactions reduce the effective particle count below the theoretical van't Hoff value, so the formula overpredicts the depression. For strong electrolytes it is more accurate to enter the experimentally measured effective i rather than the theoretical integer.