Why do we observe abnormal molecular masses?

Answer:

We observe abnormal molecular masses when the solute is an electrolyte and undergo either into association or dissociation.
One unit of an electrolyte compound separates into two or more particles when it dissolves and colligative properties depends on the number of solute particles.

Each NaCl unit dissociates into two ions-Na⁺ and CI^–. Thus, the colligative properties of a 0.1 m solution of NaCl should be twice as great as those of a 0.1 m solution containing a non-electrolyte, such as glucose or sucrose.