I've done many problems correct thanks to your advice, but I came to this one:

100cm^{3} of KOH solution (c=0.01mol/dm^{3}) is electrolysed for 20 hours (I=1A) using Pt electrodes. Calculate the concentration of OH^{-} ions after the electrolysis.

I figured out that the n of the OH^{-} ions remains the same-->1*10^{-3}mol. The cathode process is:

K^{-}:2H_{2}O+2e^{-}-->2OH^{-}+H_{2}

Need to calculate the mass of water that is lost during the process. nFz=It n=It/(Fz). z is again the problem. For 2 moles of water, 2 electrons are spent, so I thought that for 1 mole of water 1 electron is spent and I used that z=1. I got two times more moles than I should, to get the right answer.