The elevation of boiling point on adding one mole solute to 1000 g solvent is called molal boiling point elevation constant. It is denoted by Kb. Its unit is K kg mol-1.
The elevation of boiling point is the increase in boiling point of the solvent on adding non-volatile solute to the solvent. ∆ Tb = T3 – T2
Molality of solution –
$m=\frac{W_{1}}{M_{1}}X\frac{1000}{W_{2}}$
$\Delta T_{b}=\frac{K_{b}*W_{1}*1000}{M_{1}*W_{2}}$
$M_{1}=\frac{K_{b}*W_{1}*1000}{\Delta T_{b}*W_{2}}$
The molecular mass of the solute may be determined by the above formula