In the presence of non-volatile solute, the vapour pressure of the solution decreases. Vapour pressure should be increased to equal with atmospheric pressure, by increasing the temperature. This leads to an elevation in the boiling point. Elevation in boiling point is measured as,
Where ∆Tb is the elevation in boiling point
Kb is the boiling point elevation constant,
m is the molality of the solution
But on the addition of a volatile solute, vapour pressure increases. In this case, to equal vapour pressure to atmospheric pressure temperature needs to be brought down which decreases the boiling point.
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