Raoults Law for Boiling Point Elevation

9 Colligative Properties of Nonvolatile Solutes 20.1.9.1 Raoult s Law for Boiling Point Elevation 

If we consider a solution of a solvent A into which a nonionic and nonvolatile solute B is dissolved (e.g., saccharose (sucrose) in water), the total vapor pressure over the solution is equal to the partial pressure of the solvent. If there is no contribution from the solute to the vapor pressure of the solution, the solution s vapor pressure is lower than that of the pure solvent and is proportional to its molar fraction, according to Raoult s law of tonometry. 

the temperature of the solution must be raised for boiling to occur. We can predict that boiling temperature increase from a consideration of the Clausius-Clapeyron equation  

The temperature difference AT = (T - T ) is generally small enough that we can reasonably replace the product T Tby Thus we can write the following equation  

In practice, it is useful to relate the boiling point elevation to the molality of the solute, i.e., the amount of substance B per mass of solvent A expressed in mol.kg, with a good approximation for dilute solutions that consists of writing  

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