Boiling Point Elevation

Raising a solvent's boiling point by adding solute

The Formula

ΔTb = i Kb m
Tb,solution = Tb,pure + ΔTb

Variables

  • ΔTb = boiling point elevation (C)
  • i = van't Hoff factor
  • Kb = ebullioscopic constant (C·kg/mol)
  • m = molality (mol solute / kg solvent)

Typical Kb Values

  • Water: 0.512 C·kg/mol
  • Ethanol: 1.20 C·kg/mol
  • Benzene: 2.53 C·kg/mol

Step-by-Step Example

Problem:

What is the boiling point of a solution made by dissolving 1.00 mol glucose (i = 1) in 2.00 kg water? Kb water = 0.512 C·kg/mol.

1) Molality

m = 1.00 mol / 2.00 kg = 0.500 m

2) Calculate ΔTb

ΔTb = i Kb m = 1 × 0.512 × 0.500 = 0.256 C

3) New boiling point

Tb,solution = 100.0 C + 0.256 C = 100.26 C

Answer:

The solution boils at 100.26 C (ideal assumption).

Common Mistakes to Avoid

Using molarity

Use molality (mol/kg solvent), not molarity.

Ignoring i

Include van't Hoff factor for electrolytes.

Wrong constants

Use Kb for the correct solvent at the given temperature.

High concentration

At high m, deviations occur and i drops below ideal.

Related Calculators

Freezing Point Depression

ΔTf = i Kf m

Osmotic Pressure

Π = i M R T

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Frequently Asked Questions

Why use molality instead of molarity?

Molality is independent of temperature because mass does not change with thermal expansion.

What is i for CaCl2?

Ideal i = 3, but effective i is typically 2.5-2.7 at moderate concentration.

Does pressure affect ΔTb?

For typical lab conditions, pressure effects are negligible; use Kb at the measured pressure.