How to get van't Hoff Factor and Freezing Point

Question

Kf (°C/m) of water	1.86
Freezing Point of Water, °C	0.00
SOLVENT	100.0 g water

Mass / Volume added	Added reagent	Molecular Weight	van't Hoff Factor	m	Freezing Point
20.0 g	NaCl	58.44	Answer 1	Answer 2	Answer 3
25.0 g	Li2SO4	109.94	Answer 4	Answer 5	Answer 6
10.0 mL	0.150 M ethanol	-	Answer 7	Answer 8	Answer 9

Answer 1

The freezing point depression is = ik_fm where i is the van't Hoff factor, k_f is the FPDC for water and m is the molality of the solute in the water (if dilute, molality and molarity are approximately equal - not the case here)

In the ideal case, the only thing that matters is the number of particles put into solution by the solute:

For the three cases listed i = 2, 3, and 1 . In reality, especially for concentrated solutions, the ions are attracted to each other within the solution and the effect of the first salt, for example, could be 1.8. You can only determine these experimentally or find a table of van't Hoff factors for a give concentration.

Convert 20 g of NaCl to moles: 20 g * 1mole/58.44 g NaCl = moles NaCl

Find Molality = moles NaCl / .1 liters of water

ΔT_FPD = 2k_FPDm and T_FP = 0 - ΔT_FPD

The third problem requires going from molarity to molality which is problematic as the volumes do not add when alcohol and water mix. You need the density of the solution to get the molality. Probably you are supposed to assume molality = molarity in this case.