Question

Assuming equal concentrations and complete dissociation, rank these aqueous solutions by their freezing points. K2SO4 NH4I CoCl3

Answer 1

Assuming complete dissociation in aqueous solution Implies the following:

1) k2so4 (aq)--> 2K+ so4^-2
2) NH4I (aq)---> NH4+ I
3) CoCl3 (aq) ---> Co^+3 + 3Cl

So now these are considered colligative properties meaning that it doesn't matter the identity of your molecule only how much stuff solute you dissolve into a solvent. If you dissolve something in a solvent in this case water, it lowers the freezing point.

I guess one way to think about it is by figuring out how many mole of ions produced by each reaction
Van hoff factor

Reaction one we have 3 mole total
Reaction two we have 1 mole I, 1 mole NH4 so 2 mole of ions
Reaction 3 we have 4 mole total ions.

The reaction that would lower the freezing point the most would produce the most ions. So reaction 3 would have the lowest freezing point followed by reaction 1 and then reaction 2.