Introduction:
The molecular mass of a solute can be determined by measuring colligative properties, which are properties that depend on the number of solute particles present in a solution, rather than their identity. However, in some cases, the molecular mass determined by measuring colligative properties may be abnormal. This abnormal behavior can be attributed to the association or dissociation of solute molecules.

Explanation:
When a solute dissolves in a solvent, it can exist as individual molecules or can associate or dissociate to form larger or smaller clusters of molecules. This behavior is dependent on the nature of the solute and solvent. In the case of solutes that exhibit association or dissociation, the molecular mass determined by colligative properties may be abnormal.

Association:
Some solutes have a tendency to associate with each other when dissolved in a solvent. This association can occur through intermolecular forces such as hydrogen bonding, dipole-dipole interactions, or van der Waals forces. As a result, the solute molecules form larger clusters in the solution, effectively increasing the number of solute particles. This leads to a higher colligative property value and a molecular mass that is higher than expected. For example, if a solute associates into dimers in a solution, the colligative properties would suggest that there are twice as many solute particles present, resulting in a higher molecular mass.

Dissociation:
On the other hand, some solutes can dissociate into smaller particles when dissolved in a solvent. This dissociation typically occurs when the solute is an ionic compound, such as salts. When the ionic compound dissolves, the positive and negative ions separate and become surrounded by solvent molecules. This leads to an increased number of solute particles in the solution, resulting in a higher colligative property value and an abnormal molecular mass. For example, if a salt dissociates into two ions when dissolved, the colligative properties would suggest that there are twice as many solute particles present, resulting in a higher molecular mass.

Conclusion:
In conclusion, the abnormal molecular mass determined by measuring colligative properties in the case of some solutes can be attributed to the association or dissociation of solute molecules. When solute molecules associate or dissociate in a solution, the number of solute particles increases, leading to an abnormal increase in colligative property values and an abnormal molecular mass.