[Judy]

Consider an electrochemical cell with a zinc electrode immersed in 1.0M Zn 2+ and a nickel electrode immersed in 0.10 M Ni 2+.

Zn 2+ + 2e-  → Zn   E° = -0.76 V
Ni 2+ + 2e-   → Ni    E° = -0.23 V

Calculate the concentration of Ni2+ if the cell is allowed to run to equilibrium at 25°C.

A) 1.10 M
B) 1.20 M
C) 0.10 M
D) 0 M
E) none of those

The answer is E


Here's my calculation:
Zn + Ni2+ -> Zn2+ + Ni
0.53 = 0.059/2*log([Zn2+]/[Ni2+])
Since only the initial concentrations of both ions are given and the zinc ion concentration at equilibrium isn't given, it isn't possible to calculate nickle ion concentration at equilibrium. Is my reasoning correct? Thanks.

[Borek]

Hint: they react till the E of the cell equals 0. That's where the equilibrium lies.

[Judy]

Hint: they react till the E of the cell equals 0. That's where the equilibrium lies.
[/quote]
Thanks for your reply. I don't quite get the hint. Based upon my calculation, there are two variables (concentration of zinc and nickle ions), so the answer is E?

[Borek]

You have two unknowns and two equations describing their dependence: one is the Nernst equation (E=0), the other is stoichiometry - one metal gets reduced, the other oxidized, reaction equation will tell you how amounts of reacting metals are related.