Example of Dissociation Constant:

Example: The   hydroxyl   concentration   [OH^-]   of   a   water   solution o at   25ºC   is 7.2 x 10^-9 moles/liter.  Calculate the pH of the solution.

Solution:

Kw =[H+] [OH-]

[H⁺] = K_w/[OH^-]                                                       pH = -log [H⁺]

= 1× 10^-14/7.2 × 10^-9                                                   = -log (1.38 × 10^-6)

                                                                                    = 5.86

= 1.38 × 10^-6

At 25ºC, equilibrium exists among pure molecular water and its ions. The [H⁺] equals the [OH^-] and both have values of 1 x 10^-7 moles/liter. By using the pH definition, it follows which the pH of pure water at 25ºC is 7.  pH values less than 7 denotes an acidic solution and values greater than 7 denotes a basic or alkaline solution.

Figure: Ion Product Constant for Water