[Fmeub]

This will hopefully be the last question from this paper for which I don't have the memo.

Question:

You have a solution of 50.00 mL of 0.01 M AgNO₃.

a) What is the change in the potential ΔE at 25 ºC after the addition of 50.0 mL of a titrant, a 0.05 M Cl^- solution?
K_sp(AgCl) = 1.82 x 10^-10 and Eº_{(Ag+ / Ag)} = 0.799 V.

b) What is the change in the potential ΔE at 25 ºC after the addition of 50.0 mL of 0.05 M ligand solution that forms AgL₂? The overall stability constant, as log β = 16.3.

My answer for a) is 0.476 V.

The concentration of Cl^- I found to be 0.02 M which is in excess. Substituting this into [Ag⁺][Cl^-] = 1.82 x 10^-10 gives [Ag⁺] = 9.1 x 10^-9 M.

Using the Nernst equation, the change in potential is 0.476 V. 


My final answer for b) is 0.867 V, but it doesn't make sense to me.

As in a), the Ligand is in excess, and so the [L] is 0.015 M.
Since another equation is necessary, I thought that [AgL₂] + [AgL] + [Ag⁺] = 0.01 M. Assume that [Ag] and [AgL] << [AgL₂], so [AgL₂] = 0.01 M.

Substituting this into [AgL₂] / [Ag⁺][L]² = 1.995 x 10¹⁶, I get [Ag⁺] = 2.23 x 10^-15 M.

Putting this value into the Nernst equation as in a), the change in potential is 0.867 V.

What I don't understand is whether the potential from a positive value can go to 0.00, and then negative. Does it mean that the reaction "stops" and then goes in the reverse direction?