Kb=R×T2b×M1000×ΔHvap
where,
Tb is boiling point of pure solvent =100∘C=373 K
M is molar mass of water=18 g mol−1
ΔHvap is the molar enthalpy of vapourisation of water =9700 cal mol−1
R is ideal gas constant=1.987 cal mol−1 K−1
Kb=1.987×(373)2×181000×9700Kb=0.513 K kg mol−1