Freezing Point Depression Calculator
Calculate how adding a solute lowers the freezing point of a solvent. Determine new freezing point and boiling point for any solution.
Select the solvent
Molality of solute in mol/kg
What is freezing point depression?
Freezing point depression (ΔT_f) occurs when a solute lowers a solvent's freezing point. The formula: ΔT_f = i·K_f·m, where i = van't Hoff factor (particles per formula unit), K_f = cryoscopic constant (unique to each solvent), m = molality. Adding solute disrupts crystal formation.
How do you calculate freezing point depression?
ΔT_f = i·K_f·m. For nonelectrolytes, i = 1. For NaCl in water: i = 2 (Na⁺ + Cl⁻). So ΔT_f = 2 × 1.86°C·kg/mol × 0.5 mol/kg = 1.86°C. Pure water freezes at 0°C; with 0.5 m NaCl: -1.86°C.
What is the cryoscopic constant?
The cryoscopic constant (K_f) depends only on the solvent: K_f = R·T_f²·M/(ΔH_fus·1000). Higher K_f means more freezing point depression. Water: 1.86°C·kg/mol; benzene: 5.12; camphor: 40.0. Camphor is excellent for molar mass determination.
How is freezing point depression used?
Used to determine molar mass of unknown solutes (cryoscopy). Measure ΔT_f, calculate molality, then M = mass solute / (molality × kg solvent). Also explains salt on icy roads (lowers freezing point), antifreeze in cars (ethylene glycol), and sea water freezing below 0°C.