Standard Reduction Potential?

Question

What is the standard reduction potential of the Y3+/Y redox couple if the cell potential of the following cell at 25 degree C is 1.28 V?

Zn / Zn2+ (0.20 M) // Y3+ (1.2 M) / Y

Answer 1

Let the standard reduction potential of the Y3+/Y redox couple be F (V).
So the related reduction potentials are:
Y(3+) + 3e- ==> Y(s), E° = F V
Zn(2+) + 2e- ==> Zn(s), E° = -0.763 V
Thus the redox equation is:
2Y(3+) + 3Zn(s) ==> 2Y(s) + 3Zn(2+), E° = F+0.763 V

Using Nernst equation:
E = E° + (0.0591/n)*log([Y(3+)]^2/ [Zn(2+)]^3)
1.28 = F+0.763+(0.0591/6)*log(1.2^2/0.20^3)
F = 1.28-0.763-(0.0591/6)*log(1.2^2/0.20^3)
= 0.49 (V)
Ans: the standard reduction potential of the Y3+/Y redox couple is 0.49V.