Freezing Point Depression Calculator
Calculate how much the freezing point of a solvent decreases when a solute is dissolved in it. Uses the colligative property formula ΔTf = i × Kf × m, where i is the van't Hoff factor, Kf is the cryoscopic constant, and m is molality.
Freezing point depression is a colligative property describing how the addition of solute to a solvent lowers the temperature at which the solution freezes. Like boiling point elevation, it depends only on the number of dissolved particles, not the nature of the solute.
The Formula: ΔTf = i × Kf × m
- i = van't Hoff factor (number of particles per formula unit upon dissolution)
- Kf = cryoscopic constant (unique to each solvent). Water: 1.86 °C·kg/mol, Benzene: 5.12 °C·kg/mol
- m = molality (moles of solute per kg of solvent)
- The new freezing point = normal freezing point − ΔTf
Practical Applications:
Road salt (NaCl or CaCl₂) lowers the freezing point of water on roads, preventing ice formation. Automotive antifreeze (ethylene glycol) depresses the freezing point of engine coolant to well below 0 °C. The cryoscopic method is a classical technique for determining the molecular weight of an unknown solute by measuring how much it depresses the freezing point.
Comparison of Solvents:
Cyclohexane has a very high Kf (20.0 °C·kg/mol), making it especially sensitive for molecular weight determination. Benzene (Kf = 5.12) is moderately sensitive. Water (Kf = 1.86) shows smaller depressions but is the most practically important solvent.