Choose the aqueous solution below with the lowest freezing point. These are all solutions of nonvolitile solutes and you should assume ideal van't Hoff factors where applicable.

Question

a. 0.075 m NaI
b. 0.075 m (NH4)3PO4
c. 0.075 m NaBrO4
d. 0.075 m LiCN
e. 0.075 m KNO2

I can't find enough information on this subject to make it clear?

Answer 1

A little reasoning is what you need.
The freezing point is
delta T = i*kf*m
You know Kf is the same for all of them. You know, from the problem, that m is the same (0.075m) for all of them, so what's the difference? The only difference is i. A small i gives a smaller delta T than a large i, right?
Note: You COULD calculate the answers for all given then you would KNOW the freezing point of each and you could choose that way but there is no need to do that. For example, A is
i*Kf*m = 2*1.86*0.075 = 0.279 so freezing point is -0.279 degrees C.
The easy way is to use i. Largest i gives largest delta T and that gives smallest (lowest) freezing point.