pH value increases as [H+] value decreases; pH= -log([H+]) = -log(10-8) =8 ; we know that, [H20]=[H+][OH-]=[10-14] applying -log on both sides, pH + pOH =14 there fore, 1) when pH=5 , pOH=9 2)when pH=8 , pOH =6 Solution pH value increases as [H+] value decreases; pH= -log([H+]) = -log(10-8) =8 ; we know that, [H20]=[H+][OH-]=[10-14] applying -log on both sides, pH + pOH =14 there fore, 1) when pH=5 , pOH=9 2)when pH=8 , pOH =6.