Since the boiling point is defined as that temperature at which the vapor pressure equals the atmospheric pressure, anything that lowers the vapor pressure obviously will raise the boiling point. In terms of molar free energies or escaping tendencies, adding sugar molecules to boiling water at 100�C dilutes the H20 molecules, lowers their escaping tendency, and causes the boiling to cease.

To make the solution boil again, we must raise the temperature until the escaping tendency of the remaining H20 molecules is as great as before. We can set up a free energy expression that tells how the escaping tendencydepends on concentration and temperature, and look for conditions under which these two effects cancel. The result for dilute solutions, in which interactions between solute molecules or ions can be neglected, is that the increase in boiling point, DT_B, is proportional to the solute concentration expressed as molality, or number of moles of solute particles per kilogram of pure solvent:

m_A = molality of A

m_A= moles of A per kilogram of pure solvent B

DT_B=k_bm_A

The proportionality constant, k_b, varies from one solvent to another but is completely independent of the nature of the solute particles, A. The solute exerts its effect only by virtue of the number of molecules or ions present. As with vapor pressure, salts that produce several ions per molecule are more effective than molecules that do not dissociate.

Example. The molal boiling point elevation constant for water is k_b =0.512. What is the boiling point (T_b) of a solution of 0.10 mole of glucose in 1000 g of water?

Solution

Example. What is the boiling point of a 0.10-molal solution of NaCl ?

Solution