For dilutesolutions the elevation of boiling point (ΔTb) is directly proportional to the molal concentration of the solute in a solution. Thus
ΔTb ∝ m 
or ΔTb = K_bm
Here m (molality) is the number of moles of solute dissolved in 1 kg of solvent and the constant of proportionality, Kb is called Boiling Point
Elevation Constant or Molal Elevation Constant . The unit of Kb is K kg mol-1.