What Is The Hydroxide-Ion Concentration For The PH Values 4.00, 8.00 And 12.00?

PH = 4; [H3O⁺]= 10^-4
pH = 8; [H3O⁺]= 10^-8 pH = 12; [H3O+]=10^-12

Ph 11.4 as its near alkaline because of oxygen

Hydroxide concentration i.e. [OH-]  of the pH=4,8 &12 can be calculated using the following two formulae:  1) pH = - log [H+] where, [H+] represents the Hydrogen ion concentration.    Using this formula, [H+] can be calculated.  Which will be  for pH=4, [H+] = 1x10^-4  for pH = 8,[H+] = 1x10^-8 and  for pH =12, [H+] = 1x10^-12.    Now to find the [OH-] for the three solutions, we have to use another formula:  2) For any aqueous solution  [H+] x [OH-] = 1x10^-14  (This is known as the ionic product of water and is represented as Kw).    Thus for the 1st solution where  pH=4, [H+]=1x10-4  so the  [OH-] will be = 1x10^-10.  Similarly, for pH=8, [OH-]=1x10^-6 and  for pH=12, [OH-] = 1x10^-2.