Question

How do ionic solutes affect the boiling point?

Answer 1

Ionic solutes raise the boiling point more than nonionic solutes at the same concentration do.

Boiling point elevation is a colligative property. It depends only on the number of particles in the solution.

Solute particles are distributed throughout the solution. They "get in the way" of the solvent particles when the solvent wants to evaporate, so we must heat the solution to a higher temperature to make it boil.

The formula for boiling point elevation is

`ΔT_b = iK_b m`

where `m` is the molality of the solution, `K_b` is the molal boiling point elevation constant for the solvent, and `i` is a number related to the number of particles the solute contributes to the solution (the van’t Hoff `i` factor).

The van't Hoff `i` factor is a number that tells you how many moles of solute particles you get from 1 mol of solute.

Non-electrolytes don't dissociate when they dissolve. Thus, one mole of glucose will have one mole of particles in solution, and `i` = 1.

`ΔT_b = K_b m`

NaCl dissociates into Na⁺ and Cl⁻ in water. So if you have 1 mol of NaCl, you'll have 2 mol of particles and i= 2

`ΔT_b = 2K_b m`

For CaCl₂, i = 3, for FeCl₃, I = 4, etc.

Thus, a 1 mol/kg solution of FeCl₃ will raise the boiling point of water 4 times as much as a 1 mol/kg solution of glucose.

EXAMPLE

Calculate the boiling point of a 0.15 mol/kg aqueous solution of sodium chloride. `K_b` for water is 0.512 °C·kg·mol⁻¹.

Solution

`ΔT_b = iK_b m` = 2 × 0.512 °C·kg·mol⁻¹× 0.15 mol·kg⁻¹ = 0.15 °C

`T_b = T_b^o + ΔT_b` = 100.00 °C + 0.15 °C = 100.15 °C