for a question it is given that there is 14.6 g of calcium chloride in 246 g of water. Kf is 1.86 C/m

Question

we need to find the freezing point of this solution.

when i do the calculations,

i get the freezing point to be -0.99 (without using the vant hoff factor i)

or i get -2.98 (using the van't hoff factor i for CaCl3 = 3)

which one is the correct answer and why? when do i use the van't hoff factor and when do i not?

please explain, thank you

Answer 1

The -2.98C using 3 for i is the correct value to use. In truth you ALWAYS use the van't Hoff factor of i.
The equation is delta T = i*Kf*m
The van't Hoff factor is always 1 for almost all organic compounds that do not ionize. Since they don't ionize there is 1 particle/molecule so a 1 always goes in for i. Since it's a 1 we usually don't include it.
For ALL ionic compounds, the ionization determines the value of i.
For units that ionize to give just 1 particle/molecule i = 1. But I can't think of any ionic compounds that do this.
NaCl, KBr, LiF, etc ionize to give 2; therefore, i = 2.
Na2SO4, K2SO4 etc give i = 3
K3PO4 etc give i = 4.
I think this will be what you are looking for.