Question

Context: Performed boiling point elevation lab to determine molar mass of solute dissolved in pure ethanol (solute was urea): Question: In regards to intermolecular forces, why is is the assumption that the solute is a nonelectrolyte valid here? What types of intermolecular fires are present in the solvent? For what types of solutes and in which solvent would the van't Hoff factor be larger than 1?

The only one that I think I know is the last part - electrolytes in water would have a van't Hoff factor larger than 1?

Answer 1

Colligative properties are the ratio of solute and solvent particles in solution which include vapor pressure, freezing point depression, boiling point elevation and osmotic pressure. An increase (or decrease) in the Van't Hoff factor due to associative or dissociative affects the colligative properties as it leads to an increase (or decrease) of particles in the solution.

The van 't Hoff factor is a measure of the effect of a solute upon colligative properties such as osmotic pressure, relative lowering in vapor pressure, boiling-point elevation and freezing-point depression. The van 't Hoff factor is the ratio between the actual concentration of particles produced when the substance is dissolved and the concentration of a substance as calculated from its mass. For most non-electrolytes dissolved in water, the van 't Hoff factor is essentially 1. For most ionic compounds dissolved in water, the van 't Hoff factor is not 1 but it greater than 1.

For dissociation van't hoff factor

For association van't hoff factor

When solute particles dissociate in solution, is greater than 1 (e.g. sodium chloride in water, potassium chloride in water, magnesium chloride in water).

When solute particles neither dissociate nor associate in solution, equals 1.