electrolysis of NaCl問題 急 幫幫plx

In the solution, the self-ioniztion of water gives both OH-(aq) and H+(aq) ions.
H2O(l) ≒ OH-(aq) + H+(aq)
OH-(aq) ion does NOT change to O atom and H atom.

During electrolysis, OH- ions are discharged at the anode to give water and oxygen. This is because OH-(aq) ions are oxidized (the oxidation number of O is increased from -2 to 0 in O2) by electricity. The equation is:
4OH-(aq) → 2H2O(l) + O2(g) + 4e-
Therefore, O2 is formed in the discharge of OH-(aq) ions.

On the other hand, the concentration of H+(aq) ions (not H atoms) is increased, because H+(aq) ions do not react and are left in the solution.

in the electrolysis of sodium chloride solution.
if we use concentrated one, chlorine gas will be released
in the anode and hydrogen gas will be released in the
cathode.
if we use dilute one, oxygen gas will be released
in the anode and hydrogen gas will be released in the
cathode.

2008-05-30 19:04:46 補充：
in the case of dilute sodium chloride solution,

the concentration will increase because water is hydrolysed.