when you have your 4 different equations to form 4 different possible equations, what two equations do you use for water,

Question

O2(aq) + 4H+ + 4e- ----> 2H20 (+1.23)

02 (g) + 2H2O (l) + 4e- ----> 4OH- (+0.40)

2H20 (l) + 2e- ---> h2 (g) + 2OH- (aq) (-0.83)

I'm not quite sure what you are after but I need to point out that you make a statement about four different equations but write only three. Then you ask what two equations to use (out of the three or out of four)? Anyway, the first one occurs in acid solution and the last two occur in basic solution.