Question

How does the nature of the solute affect boiling point?

Answer 1

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. 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 do not dissociate when they dissolve. Thus, one mole of glucose will provide one mole of particles, 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 have 2 mol of particles and i= 2. For CaCl₂, i = 3, for FeCl₃, i = 4, etc.

`ΔT_b = iK_b m`

EXAMPLE

Calculate the boiling point of a 0.15 m aqueous solution of sodium chloride. `K_b` for water is 0.512 °C•m⁻¹.

Solution

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

`T_b` = 100.00 °C + 0.15 °C = 100.15 °C

Here is a video of a lab examining this topic.

Here is a video which shows some sample calculations of boiling point elevation.

videos from: Noel Pauller