[DUDE778]

I cant seem to get this and i dont know why
Ce4+ + e− > Ce3+    E° = 1.72 V   
Fe3+ + e− > Fe2+    E° = 0.771 V

If a solution is prepared by mixing 5.0 mL of 0.30 M Fe2+ with 3.0 mL of 0.12 M Ce4+.
What is the potential of a platinum electrode dipped into the resulting, equilibrated, solution (relative to SHE)?

I have three chances to do this and i put..
0.949 and
2.491
I AM LOST 

[Borek]

Show how you get these values. Or at least one of them.

[DUDE778]

i added the two potentials together the first time, then for my second try I subtracted the two potentials...I don't know what's wrong..

[Borek]

You need to account for the concentrations, that's what the Nernst equation is for. You have to combine stoichiometry of the reaction with Nernst equations (for each half-cell).