Calculate how solutes change the boiling and freezing points of a solution.
Select your solvent (or choose "Custom" to enter your own Kb/Kf values), enter the molality of the solution and the van't Hoff factor. The van't Hoff factor (i) accounts for dissociation of electrolytes: i = 1 for non-electrolytes (sugar, glucose), i = 2 for NaCl (Na⁺ + Cl⁻), i = 3 for CaCl₂ (Ca²⁺ + 2Cl⁻), etc.
Colligative properties depend on the number of solute particles, not their identity. Boiling point elevation: ΔTb = i × Kb × m. Freezing point depression: ΔTf = i × Kf × m. The new boiling point = normal BP + ΔTb, and new freezing point = normal FP − ΔTf.
Worked example: Dissolve 29.2 g of NaCl (MW = 58.44, i = 2) in 500 g of water. Molality = (29.2/58.44)/0.5 = 1.0 m. ΔTb = 2 × 0.512 × 1.0 = 1.024°C. New BP = 101.024°C. ΔTf = 2 × 1.86 × 1.0 = 3.72°C. New FP = −3.72°C.
Limitations: These equations are most accurate for dilute solutions. At high concentrations, ion pairing reduces the effective van't Hoff factor, and activity coefficients deviate from unity.