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Which of the following relation is correct for boiling point elevation constant (Kb)?
Here
Tb is boiling point of pure solvent
M is molar mass of the solvent
ΔHvap is the molar enthalpy of vapourisation of the solvent
R is ideal gas constant
(All quantities to be taken in SI units)

A
Kb=R2×Tb×M1000×ΔHvap
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B
Kb=ΔHvap×M1000×R×T2b
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C
Kb=R×T2b×M1000×ΔHvap
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D
None of the above
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Solution

The correct option is C Kb=R×T2b×M1000×ΔHvap
Theory:
At B.P, the vapour pressure of the liquid becomes equal to the atmospheric pressure
Let a non-volatile solute B be added to the pure liquid A.
After addition of B, the vapour pressure of the pure solvent (A) must decrease.
On addition of a non-volatile substance to a pure liquid, the B.P of the solution becomes higher than that of the pure liquid. So, the B.P. is elevated.

If:
Tb is boiling point of solution
Tb is boiling point of pure liquid
then ,
Elevation in Boiling Point
ΔTb=TbTb
Vapour Pressure (VP) of solution is less than that of pure solvent.
Temperature, at which VP equals atmospheric pressure, is higher for solution. Boiling point of the solution is higher by an amount ΔTb.

Elevation of BP is only dependent on the solute concentration and not its nature.
From experimental observations :
ΔTb=Kb×m
Here m is molality
Kb is Boiling Point Elevation Constant or Molal Elevation Constant
It is also known as ebullioscopic constant.
Unit of Ebullioscopic constant is K kg mol1.
It is the property of the solvent and is independent of the solute.
Kb=R×T2b×M1000×ΔHvap=R×T2b1000×Lvap
Lvap is the latent heat of vapourisation of the solvent





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