Salts formed by the reactions between acids and bases in definite proportions.

Salts undergo ionization in water

The cations/anions formed by 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 process of interaction between water and cations/anions or both of salts is called hydrolysis.

The pH of the solution gets affected by hydrolysis.

The cations (e.g., Na+ , K+ , Ca2+, Ba2+, etc.) of strong bases and anions (e.g., Cl – , Br– , NO3 – , ClO4 –  etc.) of strong acids simply get hydrated but do not hydrolyse.

The solutions of salts formed from strong acids and bases are neutral i.e., their pH is 7.

The other category of salts do undergo hydrolysis.

(i)salts of weak acid and strong base e.g., CH3COONa.

(ii) salts of strong acid and weak base e.g., NH4Cl,

(iii) salts of weak acid and weak base, e.g., CH3COONH4.

CH3COONa being a salt of weak acid, hydrolysis to give CH3COOH and strong base, NaOH gets completely ionised in aqueous solution.

CH3COONa(aq) → CH3COO– (aq)+ Na+(aq)

The acetate ions thus formed undergoes hydrolysis in water to give acetic acid and OH– ions CH3COO–

Acetic acid is a weak acid (Ka = 1.8 × 10–5) remains mainly unionised in solution.

CH3COONa on hydrolysis will increase the OH– ion concentration in solution, making it alkaline.

The pH of salts of weak acid and strong base a solution is more than 7.

NH4Cl formed from a weak base, NH4OH and a strong acid, HCl,

Ammonium ions undergo hydrolysis with water to form NH4OH and H+ ions

Ammonium hydroxide is a weak base (Kb = 1.77 × 10–5) and therefore remains almost unionised in solution.

The pH of NH4Cl solution in water is less than 7.

CH3 COONH4  salt is formed from a weak acid and a weak base.

pH of such solutions is determined by their pKvalues pH = 7 + ½ (pKa  – pKb)