How do solutes affect the boiling point?

May 27, 2014

The solute raises the boiling point by an amount that depends on the number of particles it contributes go the solution.

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).

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.

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