In general, solutions have a higher boiling point than does the pure solvent. This boiling point elevation is an example of a colligative property, one which depends only on the number of moles of the solute. not the identity of that solute.

The amount of the elevation is given by

DT_b = k_bm

When working with ionic solutes in water and other polar solvents, one must be careful to take into consideration the fact that the ion concentration is higher than the concentration of the solute. For example, table salt, NaCl dissolves in water to form ions

NaCl(s) -> Na₊(aq) + Cl^-(aq)

Example 1: The normal boiling point of benzene is 80.1 ^oC. It has a boiling point elevation constant of 2.53 ^oC/m. If we make up a 0.500 molal solution of Br₂ in benzene, what is the boiling point of the mixture?

Solution: Simply use the above equation and substitute in the proper values

DT_b = k_bm

DT_b = 2.53 ^oC/m * 0.500 molal

DT_b = 1.27 ^oC

Example 2: The normal boiling point of water is 100.0 ^oC and it's boiling point elevation constant is .52 ^oC/m. If we make up a 1.00 molal solution of Na₂SO₄, what is the boiling point of the mixture?

Solution 2: Again, use the above equation but we need to be careful to account for the ions. Na₂SO₄ fragments into three pieces when it dissolves,

Na₂SO₄(s) -> 2Na⁺(aq) + SO₄^-2(aq)

DT_b = k_bm

DT_b = 0.52 ^oC/m * 3.00 molal

DT_b = 1.56 ^oC

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