The molecular weights determined by using colligative properties of substances which associate or dissociate will be abnormal and are called abnormal molecular weights. Ionic substances like NaCl, BaCl₂, AlCl₃, etc., ionize in solutions. Their colligative properties are high due to increase in number of particles when compared with solutions of non-electrolytes like glucose having equal molecular mass. When molecules of substance like acetic acid in benzene associate as dimers, trimers or polymers, the number of particles decreases and their colligative properties also decrease. The ratio of the observed colligative property and calculated colligative property is called Van’t Hoff factor i.

\[\text{i = }\frac{\text{observed colligative property}}{\text{calculated colligative property}}\]

\[\text{or, i = }\frac{\text{calculated molecular mass}}{\text{observed molecular mass}}\]

The value of Van’t Hoff factor i is greater than 1 for ionic substances while it has lower value than 1 for associated substances.

\[\frac{\text{calculated molecular mass}}{\text{observed molecular mass}} = \frac{\text{Normal number of solute particles}}{\text{Number of solute particles after dissociation or association}}\]

The depression of freezing points of 0.1 M aqueous solution of HCl, CuSO₄ and K₂SO₄ are

in the ratio 1:1:1.5

The correct answer is option 3. in the ratio 1:1:1.5.

The depression of freezing point is a colligative property, which means that it depends on the number of particles in solution, not on the identity of the particles. HCl, CuSO₄, and K₂SO₄ all dissociate in solution to form 2, 2, and 3 ions, respectively. This means that the depression of freezing point for the solutions of HCl, CuSO₄, and K₂SO₄ will be in the ratio 1:1:1.5.

For example, if the freezing point depression constant for water is 1.86°C/m, then the depression of freezing point for a 0.1 M solution of HCl will be 0.186°C, the depression of freezing point for a 0.1 M solution of CuSO₄ will be 0.372°C, and the depression of freezing point for a 0.1 M solution of K₂SO₄ will be 0.558°C.

Therefore, the answer is (3).