In this section, we will learn about the elevation in the boiling point of a pure solvent when a solute component is added to it. As we know, the vapour pressure of any liquid increases with the increase in its temperature. When the temperature of pure water kept at 25°C is increased, it starts to boil eventually after attaining the temperature of 100°C, as at this temperature the vapour pressure of the water sample becomes equal to the atmospheric pressure.
Boiling point: The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure.
We also know that the vapour pressure of any solvent decreases when some amount of solute is added to it. When some amount of non-volatile solute whose vapour pressure is zero is added to a solvent, the total vapour pressure of the solution becomes lesser than that of the pure solvent.
In the graph shown above, we can see the change in vapour pressure of a pure solvent and a solution with the increase in temperature. Here, we can see that the two samples, the solvent and the solution, will attain the condition of boiling when their vapour pressure becomes equal to the atmospheric pressure. We can see here, the pure solvent attains this condition at a temperature equal to Tb° whereas, for the solution to attain this pressure, the temperature of the sample needs to be increased by a small amount equal to ΔTb, such that,
Here the value ΔTb is known as the elevation in boiling point. The value of ΔTb for a solution depends upon the number of molecules of the non-volatile solute. It has been shown experimentally that the elevation in boiling point is directly proportional to the molal concentration of the solute in a solution.
Where, kb is the boiling point elevation constant or the ebullioscopic constant and m is the molarity of the solution that is equal to the number of moles of solute dissolved in 1 kg of solvent.
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