[Chemistry] Membrane Potential

jkh4

1. The problem statement, all variables and given/known data

Given the following half-reactions:
Ce4+ + e− → Ce3+ E° = 1.72 V
Fe3+ + e− → Fe2+ E° = 0.771 V

A solution is prepared by mixing 7.0 mL of 0.30 M Fe2+ with 8.0 mL of 0.12 M Ce4+.

Calculate [Ce4+] in the solution.

I've got the potential of a platinum electrode dipped into the resulting, equilibrated, solution (relative to SHE) to be 0.767 V, and keq of 1.1×10^16 from this equation: Ce4+ + Fe2+ ⇌ Ce3+ + Fe3+.

How do you get [Ce4+]? Why isn't that the same as the concentration posted in the question?

Thanks!