Freezing Point Depression Calculator
Calculate the freezing point depression ΔTf = Kf × m × i.
Find the new freezing point of solutions.
Useful for antifreeze, road salt, and osmometry.
Freezing Point Depression
Freezing point depression is a colligative property — it depends on the number of solute particles, not their identity.
Formula:
ΔTf = Kf × m × i
T_new = T_freeze − ΔTf
Where:
- ΔTf = freezing point depression (°C or K)
- Kf = cryoscopic constant of solvent
- m = molality (mol solute / kg solvent)
- i = van’t Hoff factor (number of particles per formula unit)
Van’t Hoff factor i:
- Non-electrolytes (glucose, sucrose): i = 1
- NaCl (→ Na⁺ + Cl⁻): ideal i = 2, actual ≈ 1.8 at typical concentrations
- CaCl₂ (→ Ca²⁺ + 2Cl⁻): ideal i = 3, actual ≈ 2.5
- MgSO₄ (→ Mg²⁺ + SO₄²⁻): ideal i = 2, actual ≈ 1.4 (ion pairing)
Kf values for common solvents:
| Solvent | Normal Freezing Point | Kf (°C·kg/mol) |
|---|---|---|
| Water | 0.00°C | 1.86 |
| Benzene | 5.50°C | 5.12 |
| Cyclohexane | 6.60°C | 20.0 |
| Camphor | 179.8°C | 37.7 |
| Acetic acid | 16.63°C | 3.90 |
Practical applications:
- Road salt (NaCl): 1 mol NaCl in 1 kg water lowers freezing point by ~3.4°C (i ≈ 1.8)
- Antifreeze (ethylene glycol): a 50/50 mix by volume protects to about −37°C
- Cryoscopy: measuring molar mass of unknown compounds by measuring ΔTf → Molar mass M = (Kf × mass_solute × 1000) / (ΔTf × mass_solvent_g)