The van't Hoff factor for KCl, KBr and KI.... different or same? Experimental and NOT Theoretical

Question

The van't Hoff factor for KCl
The van't Hoff factor for KBr
The van't Hoff factor for KI

are NOT equal to each other!!!

My student is determining them experimentally expecting the same answers
Why they are not so?

The boiling point elevation of these solutions compared to water are expected to be equal?
If not, which one is expected to have the highest Boiling point? KCl or KI?

Best Regards
Majid

Answer 1

The actual van't Hoff factor will always be LOWER than the theoretical value.  This has to do with using concentrations as opposed to activities in the theoretical values.  The higher the concentration of solute, the lower will be the actual van't Hoff factor and the larger will be the discrepancy compared to theoretical values.  This is because at high concentrations, there is a greater likelihood that two ions will collide and combine into a formula unit.

 

As for a comparison of KCl and KI, the I^- ion is larger than the Cl^- ion and so would be expected to be less strongly bonded to the K⁺ ion.  From this, one would expect the van't Hoff factor for KI to be slightly greater than that for KCl, and following from this, it would expected to raise the b.p. more than KCl, assuming equal concentrations of the 2 salts.