[formaldehyde23]

This is a question that I'm really struggling with (specifically part b):

A galvanic cell is based on the following half reactions:
Ag⁺ (aq) + e^-    Ag (s) E = 0.80 V
Cu²⁺ (aq) + 2e^- Cu (s)  E = 0.34 V

In this cell, the silver compartment contains a silver electrode and excess AgCl(s) (K_sp= 1.6*10^-10) and the copper compartment contains a copper electrode and [Cu²⁺] = 2.0 M

a) Calculate the potential for this cell at 25 deg C.
This part I got pretty quickly. I just used Nernst equation; b/c Ag is the better oxidizing agent, it is the cathode; on the other hand, Cu is the anode to get an answer of .1617 V.

b) Assuming 1.0 L of 2.0 M Cu²⁺ in the copper compartment, calculate the moles of NH₃ that would be added to give a cell potential of 0.52 V at 25 deg C (Assume no volume change on addition of ammonia).

Cu²⁺ (aq) + 4NH₃ (aq) [Cu(NH3)₄]²⁺ (aq) K = 1.0*10¹³

[Borek]

Start calculating necessary concentration of Cu²⁺.

[formaldehyde23]

Yes, I used the Nernst equation again: I got [Cu2+] = 1.49*10^-12 M.

How can I proceed?

[Borek]

That means almost all copper should be in the complexed form. Just plug these concentrations into the stability (formation) constant formula and solve for ammonia.

[Siviscious]

I get 1.7*10^-2 moles....

[Borek]

No idea how you got it and it doesn't look like a correct answer at all.