[jaydee54]

Zn(s) | ZnSO4(0.0025M) || KIO3(0.10M) | Cu(IO3)2(s) | Cu(s)

The EMF of this cell is 1.033V at room temperature.
Using the Nernst Equation calculate [Cu^2+] in the cathodic compartment.

Attempt at solution:
Cu2+ + 2e- -> Cu(s)                   +0.34V
Zn(s) -> Zn2+ + 2e-                   +0.76V
----------------------------------------
Cu2+ + Zn(s) -> Cu(s) + Zn2+      +1.10V


1.033 = 1.10 + (0.05916/2)log(0.0025/x)
where x = [Cu^2+] = 1.36e-5 M
which is wrong.
What am I doing wrong?

[Hunter2]

I got 0.46 mol/l, if I type your values.

[Jack Bauer]

New member - just registered for this question 

Where did you get this question?  As far as I can see it is impossible as you have labelled the Cu ion as a solid.  Therefore it has an activity of 1 in the nernest equation.  More to the point, you can't get a concentration from a solid. 

Finally

Cu2+ + 2e- -> Cu(s)                   +0.34V

this reduction potential is not the one you want as it will change with counter ion and state of the Cu species.  Consider changes in free energy upon changing the Cu environment then consider that deltaG = -nFE

Please let me know if you have written the question wrong or if I am wrong as I am interested now.

[Hunter2]

The copper iodate has of course in aqueous solution. Otherwise the reaction will not work at all.

[Jack Bauer]

labelling should have been (aq), not (s)  

Also, it is perfectly valid to have two solids in a redox couple at an electrode, typically with a counter ion coming in and out to charge balance, although again this should be shown in the original cell.  In this case KIO3(0.10M).

If it is aq;

Mistake must be with the maths then and hunters answer will most likely be correct.

Quick Edit: sry, didnt notice it was hunter replying

[Jack Bauer]

Had a bit google of this and it seems to be a typical question.

Cu(IO3)2(s) has a low solubility and the measurement of the cell potential is used to determine the solubility constant etc.  

Solid Cu(IO3)2(s) is added to the cell compartment, but it is not in contact with your electrode.  As Above, you need Cu(IO3)2(aq) and roughly (just from values online) this has a low concentration in the mM region.

Ideally KIO3 should also be included in the nernest equation, as IO3 takes part in the equilibrium but it seems to be left out most of the time - I guess to make it simpler.

[jaydee54]

The question gives Cu(IO3)2 as a solid and not aqueous. If IO3 is included in the equation would it be in the denominator?

[Jack Bauer]

No, I believe it would be the numerator in this case due to;

Cu(Io3)2 + 2e- -> Cu(s) + 2IO3-

Although I always forget if it is ox/red or red/ox so check it 

It says solid in the question as solid Cu(Io3)2 is added to the cell, this dissolves via;

Cu(IO3)2 > Cu2+ + 2IO-   

Actually, looking at this and thinking about it you would only need IO- in the Nernest equation is Cu(IO3)2 was taking part as a solid and that is why it is not there.

[LiquidSword]

Hmm this is tricky. Is it possible to have to use activity coefficients to calculate the concentration of [Cu²⁺]?

Actually that would be to solve for KSP once you've already solved the concentration.  Hmm.

[Borek]

In this context (s) means solid being present in the solution, which most likely means you are expected to use K_sp taken from tables to calculate concentration of Cu²⁺.