Kc = 1.67e20, Fe^3+ + 3C2O4^2- = [Fe(C2O4)3]^3-?

Question

If 0.01 M Fe3+ is initially mixed with 1.00 M oxalate ion, what is the concentration of Fe3+ ion at equilibrium? THANKS

Answer 1

You don't specify the volumes mixed so how can we know the concentrations after mixing? Assuming that the concentrations just after mixing are the ones you give then

.. .. .. . .. .. .. Fe(3+) + 3C2O4(2-) <=> [Fe(C2O4)3](3-)
Initial.. .. .. .. 0.01 .. .. .. .. 1.00
React .. .. .. .. x .. .. .. .. .. 3x
Produce .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. x
At Equil. .. 0.01-x .. .. .. .. 1-3x .. .. .. .. .. .. .. . x

K=[ [Fe(C2O4)3](3-)] / [Fe(3+)][C2O4(2-)]^3 =>
K= x/(0.01-x)(1-3x)^3

Kc is so huge and the excess of oxalate as well, that the concentration of Fe(3+) is practically zero.
In detail, you have 0.01 M Fe so x<0.01 and it is safe to assume that 1-3x=1 since even the maxium difference (1-0.03) is practically 1.
then K=x/(0.01-x) => x=0.01K/(1+K) but K is 20 orders of magnitude bigger than 1, so 1+K=K and x=0.01

Also as an example, if you say that x=9.9999999999999*10^-3 then Kc=1.1*10^14 and [Fe(3+)] =10^-16 M =1 fM which means that the [Fe(3+)] << 1 fM to have Kc=1.67*10^20 and doesn't make much sense to calculate.

You could try to solve the 4th degree polyonymial but I don't think you'll get a better answer.