Question

Kc is 1.67 × 1020 at 25°C for the formation of iron(III) oxalate complex ion:
Fe3+(aq) + 3 C2O42-(aq) [Fe(C2O4)3]3-(aq).
If 0.0200 M Fe3+ is initially mixed with 1.00 M oxalate ion, what is the concentration of Fe3+ ion at equilibrium?

Answer 1

This is a problem that almost has to be worked "backwards" to make it work.
.....Fe^3+ + 3C2O4^2- ==> [Fe(C2O4)3]^3-
I....0.02....1.00...........0
C....-0.02..-0.06..........0.02
E......0.....0.94..........0.02

What I have done above is to look at the size of Kc (a huge number) and assume the reaction will go essentially to completion. I know Fe^3+ won't be zero at equilibrium although it will be very very small; therefore, you can't calculate how much is left because all most all of it is gone. The way you handle a problem like this is to let it go to completion, then take the E line amounts and let those be initial amounts in another ICE chart and that way the small number can be calculated. Like this.
....Fe^3+ + 3C2O4^2- ==> [Fe(C2O4)3]^3-
I.....0.......0.94..........0.02
C.....x.......3x.............-x
E.....x......0.94+3x.......0.02-x

Then substitute the E line into the Kc expression and solve for x = (Fe^3+)

Answer 2

867687