What do you mean by abnormal molecular mass ? Why do we gets abnormal molecular masses from colligative properties

Explanation

We know that colligative properties depend upon the molar or molar concentration of solute in solution. These properties depend on the number and not upon the nature of solute particles in the solutions. Thus, in may cases where the solute dissociate or associate in solution, abnormal values of colligative properties and hence abnormal molecular mass result .
(i) Dissociation . When a solute dissociates in solution, the number of solute particles in the solution increases. Therefore colligative property increases. for example in a dilute acqueous solution of sodium chloride almost whole of sodium chloride dissociates and exists as `Na^(+) and Cl^(-)` ions . Thus the observed value of the colligative property of such a solution has higher value than the calculated one
`NaCl overset("water") to NA^(+) (aq) = C,^(-1) (aq)`
As molecular mass is inversely proportional to colligative property, an increase in the value of colligative property decreases the value of observed molecular mass
(ii) Association . Due to association of solute particles in solution, the number of solute particles present the solution will be less then the actual number dissolved. Thus the observed value . Thus the observed value of colligative properties will be less than calculated one
As molecular mass is inversely proportional to the magnitude of the colligative property, the observed molecular mass will be more than calculated one.
For example , benzeoic acid associate in benzene to form dimers due to hydrogen bending and its observed molecualr mass is found to be more than the calculated one