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.