[Rutherford]

I get the right answer if I use 2 electrons.

[Olympiad_Tutor]

originally you said that "For 2 moles of water, 2 electrons are spent" and that you "got two times more moles than I should, to get the right answer"

now we figured out that for 2 moles of water, 4 electrons are spent and you still get the wrong answer.  this seems suspect.

[Rutherford]

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.

I used that z is 1 first time.