Abnormal molecular masses are experimentally determined when association or dissociation takes place.A number equal to the sum of the atomic masses of the atom in a molecule. The molecular mass gives the mass of a molecules relative to that of the ¹²C atom, which is taken to have a mass of 12.
When the molecular mass of a substance as determined by using colligative properties doesnot come out to be same as expected theoretically, then it is said to show abnormal molecular mass.  Abnormal molecular mass is obtained when the substance in the solution undergoes dissociation or association.  Dissociation results in the increase in the number of particles hence increase in the value of colligative property in the value of colligative property and decrease in molecular mass as all the colligative properties are inversely proportional to molecular mass.  Association results in the decreases in the number of particles and hence decrease in the value of colligative property and increase in the molecular mass.  Electrolytes in water undergo dissociation, while in some non-polar solvents, two or more molecules of solute "associate to form bigger molecules, e.g. Acetic acid in benzene and benzoic acid in benzene undergo association to form dimers. This association is due to the formation of hydrogen bonding

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