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Question

(a) Define an acid salt and a normal salt.
(b) How many salts can be obtained from orthophosphoric acid? Is there any difference in the salts formed by the acid?

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Solution

(a) An acid salt is formed by the partial neutralisation of dibasic or tribasic acids with replaceable hydrogen atoms present in them. For example, NaHSO4 is formed by the reaction between dibasic sulphuric acid and sodium hydroxide. ​
NaOH + H2SO4 → NaHSO4 + H2O
A normal salt is formed by the complete neutralisation of monobasic acid, with no replaceable hydrogen atoms present in it. For example, NaCl is formed by the reaction between monobasic hydrochloric acid and sodium hydroxide.
NaOH + HCl → NaCl + H2O

(b) Three salts can be obtained from orthophosphoric acid (H3PO4). Orthophosphoric acid is a tribasic acid; it forms three hydronium ions per molecule when ionised in water. This tribasic acid has three replaceable hydrogen atoms that form three types of salts (two acid salts and one normal salt) when reacted with a base.

Ionisation of orthophosphoric acid:

H3PO4 + H2O H2PO4- Dihydrogen phosphate ion H2PO4- + H2O HPO42- Monohydrogen phosphate ion HPO42- + H2O PO43- Phosphate ion


Reaction with base:


H3PO4 + NaOH NaH2PO4 + H2O Acid salt (Monosodium hydrogen phosphate ion) H3PO4 + 2NaOH Na2HPO4 + 2H2O Acid salt (Disodium hydrogen phosphate ion) H3PO4 + 3NaOH Na3PO4 + 3H2O Normal salt (Sodium phosphate)

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