i is the Vant Hoff factor. It is determined by whether the solute particles in the solution associate or dissociate. If all the particles of the solute trimerise (Three associate into one) the Vant hoff factor will be 1/3. If they dissociate into two particles (For example NaCl as solute which breaks into Na+ and Cl-), i=2.
R - Ideal Gas constant
Tb - Boiling point of liquid.
Lv - Latent heat of vaporisation per gram.
This property of elevation of boiling point is a colligative property. It means that the property, in this case depends on the number of particles dissolved into the solvent and not the nature of those particles.