I've done many problems correct thanks to your advice, but I came to this one:
100cm3 of KOH solution (c=0.01mol/dm3) 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*10-3mol. The cathode process is:
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.