Question

What is the effect of the van't Hoff factor on the boiling point elevation or freezing point depression for a given electrolytic solution?

Answer 1

Well, usually `i >=1`, so in many cases, it will accentuate the boiling point elevation or freezing point depression from the pure solvent.

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For non-electrolytes, we would normally write

`DeltaT_f = T_f - T_f^"*" = -K_fm` `" "bb((1))`

`DeltaT_b = T_b - T_b^"*" = K_bm` `" "" "bb((2))`

where:

• `T_f` and `T_b` are the freezing and boiling points, respectively, of the solution. `"*"` indicates pure solvent.
• `K_f` and `K_b` are the freezing point depression and boiling point elevation constants, respectively, of the solvent.
• `m` is the molality of the solution in `"mol solute/kg solvent"`.

When we account for electrolytes, i.e. those substances that dissociate in solution, we write in the van't Hoff factor `i` to account for the effective number of solute particles per formula unit:

`DeltaT_f = T_f - T_f^"*" = -color(red)(uli)K_fm` `" "bb((1"'"))`

`DeltaT_b = T_b - T_b^"*" = color(red)(uli)K_bm` `" "" "bb((2"'"))`

And thus, `i >= 1` for typical electrolytes, increasing the magnitude to which the boiling points increase and freezing points decrease.