Self-ionisation of Water

For a strong acid, the concentration of the ions will equal the concentration of the acid. e.g. 0.1 M HC1 (aq), [H⁺] = 0.1 M

A more dilute solution will have a lower [H⁺]; e.g. 0.001 M HCl (aq), [H⁺] = 0.001M.

However for a 0.1 M solution of a weak acid such as ethanoic acid, we will find that [H⁺] « 0.1M. A more dilute solution of ethanoic acid will have an even lower [H⁺].

Self-ionisation of water:

The amphoteric nature of water is seen even in pure water. Occasionally, one water molecule can donate a proton to another, forming ions.

H₂O (I) + H₂O (aq) ↔ H₃O + (aq) + OH- (aq)

An equilibrium constant Kc expression can be written for this dissociation:

The concentration of the ions is extremely small, so the concentration of H₂O remains essentially constant. Rearranging the above equation gives:

[H₂O]² K_C = [H₃O⁺] [OH-]