Question

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×M/1000×ΔHvap
B. Kb=ΔHvap×M/1000×R×T²b
C. Kb=R×T2b×M/1000×ΔHvap
D.None of the above

Answer 1

The correct relation for the boiling point elevation constant (Kb) is Kb = R² × Tb × M / (1000 × ΔHvap), which can be derived from the Clausius-Clapeyron equation.

Step-by-step explanation:

The relation for the boiling point elevation constant (Kb) that is correct is option A. Hence, the correct relation is:

Kb = R² × Tb × M / (1000 × ΔHvap)

Where R is the ideal gas constant, Tb is the boiling point of pure solvent in Kelvin, M is the molar mass of the solvent in kg/mol, and ΔHvap is the molar enthalpy of vaporization of the solvent in J/mol. This formula can be derived from the Clausius-Clapeyron equation, which relates the vapor pressure of a liquid with its temperature. By assuming that the enthalpy of vaporization (ΔHvap) remains constant over the temperature range of interest, and rearranging the equation in terms of mole fraction, one can arrive at the formula for Kb that relates the elevation in boiling point with the molality of the solution.