formed by the reactions between acids and bases in definite proportions, undergo ionization in water. The cations/anions formed on ionization of salts either exist as hydrated ions in aqueous solutions or interact with water to reform corresponding acids/ bases depending upon the nature of salts. The later process of interaction between water and cations/anions or both of salts is called hydrolysis. The pH of the solution gets affected by this interaction.
The cations (e.g., Na + , K + , Ca + , Ba + , etc.) of strong bases and anions (e.g., Cl – , Br – , NO – , ClO – etc.) of strong acids simply get hydrated but do not hydrolyse, and therefore the solutions of salts formed from strong acids and bases are neutral i.e., their pH is . However, the other category of salts do undergo hydrolysis. We now consider the hydrolysis of the salts of the following types : (i) salts of weak acid and strong base e.g., CH COONa. (ii) salts of strong acid and weak base e.g., NH Cl, and (iii) salts of weak acid and weak base, e.g., CH COONH .
In the first case, CH COONa being a salt of weak acid, CH COOH and strong base, NaOH gets completely ionised in aqueous solution. CH COONa(aq) → CH COO – (aq)+ Na + (aq) Acetate ion thus formed undergoes hydrolysis in water to give acetic acid and OH – ions CH COO – (aq)+H O(l) CH COOH(aq)+OH – (aq) Acetic acid being a weak acid ( K a = . × – ) remains mainly unionised in solution. This results in increase of OH – ion concentration in solution making it alkaline.
The pH of such a solution is more than . Similarly, NH Cl formed from weak base, NH OH and strong acid, HCl, in water dissociates completely. NH Cl(aq) → NH + (aq) +Cl – (aq) Ammonium ions undergo hydrolysis with water to form NH OH and