For dilute solutions the elevation of boiling point (∆T_b ) is directly proportional to which of the following?

Molal concentration of solute

∆T_b ∝ m

or ∆T_b = K_b m 

Here m (molality) is the number of moles of solute dissolved in 1 kg of solvent and the constant of proportionality, K_b is called Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic Constant). The unit of K_b is K kg mol^-1.