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Salt Hydrolysis

When a salt is added to water, the solid salt first dissolves to form aqueous salt, which then dissociates into ions. The dissociation of the salt is always taken 100% unless and otherwise mentioned in the problem. The ions of the salt then may or may not react with water. The cations when react with water always produce H3O+ ions, and the anions on reaction with water always produce OH ions. Depending on the extent of hydrolysis and the amount of H3O+ and OH in the solution, the solution behaves acidic or alkaline or neutral. Let the salt be represented as BA.
 
Description: 21860.png
Description: 21869.png (anionic hydrolysis)
Description: 21876.png (cationic hydrolysis)

Salt of a strong acid and a weak base

Let us take a salt NH4Cl. It is a salt made from the reaction of strong acid HCl and weak base NH4OH.
Description: 21883.png
Out of Description: 21892.png and Cl, only Description: 21900.png ion hydrolyzes. Let its degree of hydrolysis be h, and Kh be its hydrolysis constant. Let c M be the concentration of the salt, so the concentration of Description: 21911.png will also be c M.
Description: 21918.png
 
Description: 21925.png
Description: 21933.png;
Description: 21943.png
∴ pH = 7 – Description: 26234.png pKbDescription: 26243.png log c

Salt of a weak acid and a strong base

Let us take a salt CH3CO2Na. It is a salt made from the reaction of weak acid CH3CO2H and strong base NaOH.
 
CH3CO2Na(s) CH3CO2Na(aq) CH3CO2(aq) + Na+(aq)
 
Out of Description: 21959.png and Na+, only Description: 21968.png ion hydrolyzes. Let its degree of hydrolysis be h, and Kh be its hydrolysis constant. Let c M be the concentration of the salt, so the concentration of Description: 21976.png will also be c M.
 
Description: 21985.png
Description: 21993.png
Description: 22000.pngDescription: 22008.png
∴ Description: 22015.png
∴ pH = 7 + Description: 26257.png pKa + Description: 26266.png log c

Salt of weak acid and weak base

Let us take a salt CH3CO2NH4. It is a salt made from the reaction of weak acid CH3CO2H and weak base NH4OH.
 
Description: 22025.png
 
Out of Description: 22033.png and Description: 22043.png both the ions hydrolyze. Let their degree of hydrolysis be h, and Kh be the hydrolysis constant of salt. Let c M be the concentration of the salt, so the concentration of Description: 22054.png and Description: 22064.png would also be c M.
 
Description: 22072.png
∴ Description: 22085.png
Description: 22092.png
 Description: 22099.png
Description: 22108.png
Description: 22115.png
∴ Description: 22122.png
Description: 22132.png
∴ pH = 7 + Description: 26285.png (pKa– pKb)

Salt of a strong acid and a strong base

Let us consider a salt NaCl made by the reaction of strong acid HCl and strong base NaOH.
 
NaCl(s) NaCl(aq) Na+(aq) + Cl(aq)
 
Neither of the two ions of the salt undergoes hydrolysis. So the solution contains only the equilibrium of ionization of water.
 
Description: 22142.png
 
Thus, the pH of such salts always remains constant and the solution behaves neutral at pH = 7 at 25°C.




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