Boiling-point elevation

Boiling-point elevation is the increase in temperature of the boiling point of a liquid that contains non-volatile solute. It is a colligative property: for dilute solutions of non-electrolytes, the difference in boiling point from that of the pure solvent is proportional to the amount of solute.

Ebullioscopy is the experimental technique that uses boiling-point elevation to measure the molecular weight of compounds. Tonometry is a related technique that directly measures the reduction in vapour pressure of the solution compared to that of the pure solvent. Both techniques are of historical interest only.

Description

The boiling-point elevation ΔT_b is proportional to the molality of the solution m: the proportionality constant is called the ebullioscopic constant K_eb.

ΔT_b = K_ebm

The molality of the solution is the amount of solute divided by the mass of solvent. For dilute solutions, where the mass of the solution can be approximated by the mass of solvent, the molality is equal to the mass fraction w of solute divided by its molar mass M. Hence, the molar mass of a solute can be calculated from the boiling-point elevation by

M = K_ebw/ΔT_b

See also

• Freezing-point depression