Solution.

Since 1 ml of water weighs 1 g, the solution was prepared at a strength of 30 g salt per kilogram of water. The molecular weight of potassium ferricyanide is 329 g mole^-1, so the molality is

The expected freezing point lowering of water if no dissociation takes place would be

AT_f=-1.86(0.0912) =-0.170° C

The observed depression is four times this, so potassium ferricyanide must dissociate into four ions in aqueous solution. We know from other evidence that the answer is

The potassium ions dissociate, but the ferricyanide complex remains intact as a unit. (What would the freezing point of the solution have been, had potassium ferricyanide dissociated completely in the way proposed prior to the exercise?)