In any aqueous solution at 25 °C, the sum of pH and pOH is 14.0. Explain why this is so.
Here, we are going to derive the the sum of pH and pOH is 14.0 at 25 °C.
Water is amphoteric which can act as either an acid or a base. Pure water can behave as an acid and base with itself. Thus the process is called autoionization.So, we can write,
H2O (l) --------> H+ (aq) + OH-(aq)
The Equilibrium constant for the reaction is
Kw = [H+(aq)][OH-(aq)]------(1)
where Kw is known as ion product constant for water and at 25oC.
For pure water, at 25oC, Kw = 1.0 x 10-14
Therefore, equation (1) ,
1.0 x 10-14 = [H+][OH-]------(2)