Electrochemistry - Cell Potential

[ThirdEyeBlind]

1. The problem statement, all variables and given/known data
Line Notation Fe | Fe^(2+) (0.587) || Cd^(2+) (1.009) | Cd
T = 297 K
E(cell standard) = .04 V

Find the potential when [Fe^(2+) changes by .393 M

Work
Cd^(2+) + Fe --> Cd + Fe^(2+)
Q = [Fe^(2+)] / [Cd^2+]

Q= [.587 + .393] / [1.009-.393]


3. The attempt at a solution
After I got the problem wrong and I worked backwards to get the answer but I am not sure I understand it. When finding the Q value, how do I know to add .393 M to Fe^2+ and subtract .393 M from Cd^(2+)? To me the question is vague and just says changes by .393 M, not increase or decrease so how I know [Fe^2+] must increase?

Thanks

[Borek]

I agree, questions is badly worded (or you have not quoted it properly).

I guess what they mean is that reaction in this cell will spontaneously go in one obvious direction (driven by the potential difference, or thermodynamics). That also means you can easily calculate - from the reaction stoichiometry - how much Cd must react if Fe reacts. There is a problem though: I don't know if volumes of both solutions are identical, so "add here subtract here" is in general wrong.