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, ${\mathrm{NaHSO}}_{4}$ 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 (${\mathrm{H}}_{3}{\mathrm{PO}}_{4}$). 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: ${\mathrm{H}}_{3}{\mathrm{PO}}_{4}+{\mathrm{H}}_{2}\mathrm{O}↔{\mathrm{H}}_{2}{\mathrm{PO}}_{4}^{-}\phantom{\rule{0ex}{0ex}}\mathrm{Dihydrogen}\mathrm{phosphate}\mathrm{ion}\phantom{\rule{0ex}{0ex}}{\mathrm{H}}_{2}{\mathrm{PO}}_{4}^{-}+{\mathrm{H}}_{2}\mathrm{O}↔{\mathrm{HPO}}_{4}^{2-}\phantom{\rule{0ex}{0ex}}\mathrm{Monohydrogen}\mathrm{phosphate}\mathrm{ion}\phantom{\rule{0ex}{0ex}}{\mathrm{HPO}}_{4}^{2-}+{\mathrm{H}}_{2}\mathrm{O}↔{\mathrm{PO}}_{4}^{3-}\phantom{\rule{0ex}{0ex}}\mathrm{Phosphate}\mathrm{ion}$ Reaction with base: ${\mathrm{H}}_{3}{\mathrm{PO}}_{4}+\mathrm{NaOH}↔{\mathrm{NaH}}_{2}{\mathrm{PO}}_{4}^{}+{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0ex}{0ex}}\mathrm{Acid}\mathrm{salt}\phantom{\rule{0ex}{0ex}}\left(\mathrm{Monosodium}\mathrm{hydrogen}\mathrm{phosphate}\mathrm{ion}\right)\phantom{\rule{0ex}{0ex}}{\mathrm{H}}_{3}{\mathrm{PO}}_{4}+2\mathrm{NaOH}↔{\mathrm{Na}}_{2}{\mathrm{HPO}}_{4}^{}+2{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0ex}{0ex}}\mathrm{Acid}\mathrm{salt}\phantom{\rule{0ex}{0ex}}\left(\mathrm{Disodium}\mathrm{hydrogen}\mathrm{phosphate}\mathrm{ion}\right)\phantom{\rule{0ex}{0ex}}{\mathrm{H}}_{3}{\mathrm{PO}}_{4}+3\mathrm{NaOH}↔{\mathrm{Na}}_{3}{\mathrm{PO}}_{4}^{}+3{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0ex}{0ex}}\mathrm{Normal}\mathrm{salt}\phantom{\rule{0ex}{0ex}}\left(\mathrm{Sodium}\mathrm{phosphate}\right)\phantom{\rule{0ex}{0ex}}$

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