Boiling point condition

A liquid boils when its vapour pressure becomes equal to the external pressure.

Here, from the given vapour pressure vs. temperature graph, the boiling point of the solvent is the temperature at which the solvent curve reaches atmospheric pressure.

Relation between solvent and solution curves

For a non-volatile solute:

• Vapour pressure of the solution is lower than that of the pure solvent at the same temperature.
• Therefore, the solution boils at a higher temperature than the pure solvent.

Reading the graph

From the graph, the solution reaches atmospheric pressure at a temperature higher than the solvent. The solvent curve reaches the same pressure at 82^\mathrm{C}.

Hence

The boiling point of the solvent is:

82^\mathrm{C}