JEE Revision - pH and Buffer Solutions

Ionic Product of Water

The Ionic Product of Water, Kw, is the equilibrium constant for the reaction in which water undergoes an acid-base reaction with itself. That is, water is behaving simultaneously as both an acid and a base.

H2O(l) + H2O(l) = H3O+(aq) + OH-(aq)

Kw = [H3O+(aq)][OH-(aq)]

At 298 K, the value of Kw is 1 x 10-14 mol^2 dm^-6. This makes the concentration of H+ ions equal to 1 x 10-7 mol dm^-3, and therefore the pH is 7. This is defined as 'neutral'.

From the above equilibrium expression, taking -log10 throughout

pKw = pH + pOH = 14

Ionic Product does not apply only to water. It applies, for example, to the equilibrium in liquid ammonia:

NH3 + NH3 = NH2- + NH4+


pH

pH is related to hydrogen ion concentration. Since the H+ ion concentration in solution is often small, the concentration is generally expressed as the logarithm of its reciprocal, which is called a pH value. Therefore, pH is defined as

pH = -log[H+]

For a ten times increase in H+ ion concentration there is a decrease in the pH value of one unit.

Given the pH of a solution, its H+ concentration can be found:

[H+(aq)] = antilog -pH


or

[H+(aq)] = 10^-pH