Question

How would a determination of the boiling point elevation constant be affected by forgetting to account for the fact that the compound is ionic and breaks up into 3 ions per formula unit?

Answer 1

boiling point elevation can be mathematically written as
     ΔT = i K_b m

where   ΔT is elevation in boiling point
      K_b is the molal boiling point elevation constant, and
  m is the molal concentration of the solute in the solution
   i is vantoff factor
vantoff factor :-
It is the ratio between the actual concentration of particles produced when the substance is dissolved, and the concentration of a substance as calculated from its mass.

for an ionic compound which dissociates completely
when you dissolve 1mole of any ionic compounds which is giving 3 moles of ions its vantoff factor is 3

so from equation   ΔT = i K_b m
when i increses boiling point elevation increses
but if you forget to account that your boling point elevation will be less by a factor of 3

let's say boiling point elevation when unaccounted for ions is ΔT₁
and boliling point elevation when ions are accounted is ΔT₂
can be seen as
ΔT₁ = K_b* m ---- (1)
ΔT₂ = 3*K_b* m -----(2)

dividing (1) by (2) we get

3ΔT₁ = ΔT₂