Question

What is the cause of obtaining abnormal molecular masses?

Answer 1

1. Association in solvent: In a solution, interactions between solute molecules cause them to associate. This modifies the colligative characteristics by reducing the quantity of solute particles. Any decrease in the number of particles raises the molar mass because colliding properties rely on the quantity of solute particles rather than their makeup. For example, due to hydrogen bonding, acetic acid undergoes dimerisation in non-polar solvents like benzene. Resultantly, the effective number of solute particles decreases, leading to an abnormally high molar mass. This phenomenon is more common in solvents with low dielectric constants.
2. Dissociation in solvents: Solute molecules may split up into many ions or smaller particles when they dissolve in a solvent. As a result, the solution contains more solute particles, improving its colliding qualities. An increase in solute particles results in a reduced observed molar mass since molar mass and colliding characteristics are inversely correlated. For example, when weak electrolytes like hydrofluoric acid (HF) dissolve in water, they dissociate into hydrogen (H⁺) and fluoride (F⁻) ions, leading to a decrease in the observed molar mass.