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Give a brief description of the topic hydrolysis of salt and pH of solution.

Hydrolysis of salt and pH of solution

Hydrolysis of salts results in acids and bases, depends upon the nature of the salts.

During Hydrolysis, the Cations/anions formed either exist as hydrated ion in aqueous solutions or interact with water to reform their corresponding acids and bases.

The pH of the solution gets attached by this interaction.

The cations of strong bases and anions of strong acids simply get hydreated but do not hydrolyse, and therefore the solutions of slts formed from acids and bases are neatural [pH = 7]

The other catagories of salts undergo hydrolysis.

1.Salts of weak acid and strong base CH₃ COONa.
2.Salts of strong acid and weak base NH₄Cl.
3.Salts of weak acid and weak base CH₃COONH₄

CH₃COONa; gets completely ionised in aqueous solution giving weak acetic acid and string NaOH.

CH₃COONa    ?    CH3COO^-     +   Na⁺
          (aq)                            (aq)

The acetic ion formed undergoes hydrolysis in water to give acetic acid and OH^- ions.

CH₃COO^-    +  H₂O   ?    CH₃COOH    +    OH^-
Being weak acid, remains unionised and therefore there is an increase in OH^- ions concentration in solution making it alkaline pH value > than 7.

III^ly, NH₄Cl dissociates to give NH₄⁺ and Cl^- 1ms
These NH₄⁺ ions on hydrolysis, form NH₄OH and H⁺ ions
NH₄⁺    +   H₂O  ?   NH₄OH    +    H⁺
The NH₄OH is a weak base, hence unionised, results in an increased concentration of H⁺ ion making the solution acidic.
pH value < than 7

3.Hydrolysis of CH₃COONH₄ results weak acid and weak base. Both are in solution and hence its pH is calculated by

pH  = 7 + ½  (PKa - PKb)

PKa is ionisation constant for acetic acid.

PKb  is ionisation constant for Ammonium hydroxide.