Dissociation or association of solutes causes measured molar masses to differ from expected ones.
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Abnormal molar mass is a measured molar mass of a solute that differs from the expected value because the solute dissociates or associates in solution. Colligative measurements give the number of particles, so if a solute dissociates the apparent molar mass is lower than the formula mass. If a solute associates into dimers or larger aggregates, the apparent molar mass is higher. The van't Hoff factor i is used to calculate the degree of dissociation or association from the measured property. Acetic acid is a classic example, because it forms hydrogen-bonded dimers in nonpolar solvents such as benzene.
Key ideas: Method: measure a colligative property, calculate an apparent molar mass M_obs = M_calc / i. If i is above 1, dissociation. Degree of dissociation alpha = (i - 1)/(n - 1). For dimerisation, alpha = 2(1 - i). Example: KCl with i = 1.9 gives M_obs about 39 against 74.5.
Common mistakes and connections: Students think abnormal molar mass means the experiment is wrong; it reveals actual behaviour in solution. Another error is applying the same i for association and dissociation. The ratio observed to calculated mass is the inverse of i. Benzoic acid also dimerizes in benzene. Links to van't Hoff factor, hydrogen bonding and electrolyte dissociation.
Abnormal molar mass: association such as acetic acid dimerizing in benzene (i near 0.5) gives a higher apparent molar mass than expected. The interactive lesson shows this as a live animation or chart.
Acetic acid, with a formula mass of 60, has an apparent molar mass near 120 in benzene because its molecules form hydrogen-bonded dimers.
Dissociation or association of solutes causes measured molar masses to differ from expected ones.
Abnormal Molar Mass is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in States of Matter & Solutions first: Osmotic Pressure, Reverse Osmosis, van't Hoff Factor.
The next topics in States of Matter & Solutions are Colloids and Tyndall Effect.
Yes. The interactive lesson includes a self-check question on Abnormal Molar Mass, and the States of Matter & Solutions quiz draws on every topic in this area.
Abnormal Molar Mass is topic 94 of 109 in States of Matter & Solutions.