Boiling Point Elevation Calculator
Calculate boiling point elevation of a solution from molality and the boiling-point constant.
Boiling Point Elevation (ΔTb)
Calculation Steps
Additional Information
What Are Colligative Properties?
Colligative properties are properties of solutions that depend on the number of dissolved solute particles, not on their chemical identity. Boiling point elevation is one of four classic colligative properties, alongside freezing point depression, vapor pressure lowering, and osmotic pressure. When a non-volatile solute is dissolved in a solvent, the boiling point of the resulting solution rises above that of the pure solvent.
The magnitude of the elevation depends on how many particles the solute contributes to the solution. This is why a salt that dissociates into multiple ions raises the boiling point more than the same molal amount of a non-dissociating molecular solute such as sugar.
The Boiling Point Elevation Formula
ΔTb = i × Kb × m
New boiling point = Normal BP + ΔTb
- • ΔTb = Boiling point elevation (°C)
- • i = van't Hoff factor (number of particles per formula unit)
- • Kb = Ebullioscopic (molal boiling point elevation) constant (°C·kg/mol)
- • m = Molality of the solution (mol/kg)
Ebullioscopic Constants of Common Solvents
| Solvent | Kb (°C·kg/mol) | Normal Boiling Point (°C) |
|---|---|---|
| Water | 0.512 | 100 |
| Ethanol | 1.22 | 78.4 |
| Benzene | 2.53 | 80.1 |
Understanding the van't Hoff Factor (i)
The van't Hoff factor (i) accounts for the number of particles a solute produces when dissolved. For a solute that does not dissociate, such as glucose or sucrose, i = 1. For ionic compounds that dissociate, i equals the number of ions produced per formula unit.
- • Glucose (C₆H₁₂O₆): i = 1 (does not dissociate)
- • Sodium chloride (NaCl): i = 2 (Na⁺ + Cl⁻)
- • Calcium chloride (CaCl₂): i = 3 (Ca²⁺ + 2 Cl⁻)
- • Aluminum chloride (AlCl₃): i = 4 (Al³⁺ + 3 Cl⁻)
In real solutions, the measured van't Hoff factor is often slightly lower than the ideal value because of ion pairing. The ideal whole-number value works well for dilute solutions and introductory calculations.
Note: This calculator assumes ideal, dilute solutions and a non-volatile solute. It uses the ideal whole-number van't Hoff factor and does not account for ion pairing, activity coefficients, or pressure effects. For precise work, consult experimentally measured values.