The freezing point depression is less than predicted because of the formation of ion pairs.
It dissociates in water to form 4 mol of ions for each mole of compound.
The formula for freezing point depression is
The predicted freezing point depression of the solution is
The observed freezing point depression is only
The effective concentration is also
The solution must be non-ideal.
Most likely, some of the ions exist in solution as ion pairs like
Each ion pair behaves as if it were a single particle.