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Last updated date: 28th Nov 2023
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MVSAT Dec 2023

Give two examples of tribasic acid.

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Hint: Acids release $H^+$ ions or protons when dissolved in water; the number of such ions is simply the basicity of that acid.

Complete answer:
An acid is a molecule which can donate an \[{{H}^{+}}\] ion when dissolved in water and can remain energetically favourable after doing so.
Basicity of an acid: Basicity is the number of \[{{H}^{+}}\]ions released when dissolved in water or the number of replaceable \[{{H}^{+}}\]ions present in one molecule of an acid. Suppose an acid is dissolved in water and it is producing n number per molecule of \[{{H}^{+}}\] ions then n will be the basicity of that acid.
Monobasic acid: When one molecule of an acid releases one \[{{H}^{+}}\] ion on dissolving in water, its basicity is one and it is called a monobasic acid. For example HCl:
\[HCl\to {{H}^{+}}+C{{l}^{-}}\]
Dibasic acid: When an acid releases two \[{{H}^{+}}\]ions on dissolving in water, it is called dibasic acid. For example, \[{{H}_{2}}S{{O}_{4}}\]:
\[{{H}_{2}}S{{O}_{4}}\to 2{{H}^{+}}+SO_{4}^{2-}\]
Tribasic acid: When an acid releases three \[{{H}^{+}}\] ions on dissolving in water its basicity is three and is called tribasic acid.
Examples of tribasic acids:
\[C{{H}_{2}}COOH\text{ }\text{ }COHCOOH-C{{H}_{2}}~COOH\] and \[{{H}_{3}}P{{O}_{4}}\]
\[C{{H}_{2}}COOH\text{ }\text{ }COHCOOH-C{{H}_{2}}~COOH\]or citric acid has three replaceable \[{{H}^{+}}\]ions therefore its basicity is 3 and hence it is tribasic acid.
\[{{H}_{3}}P{{O}_{4}}\to 3{{H}^{+}}+PO_{4}^{3-}\], since \[{{H}_{3}}P{{O}_{4}}\] releases 3\[{{H}^{+}}\] ions therefore it is also tribasic.

It is not always true that the number of hydrogen atoms in the chemical formula of an acid is equal to its basicity. For example, \[C{{H}_{3}}COOH\to C{{H}_{3}}CO{{O}^{-}}+{{H}^{+}}\]. There were 4 hydrogen atoms in the formula, \[C{{H}_{3}}COOH\]but only one hydrogen acid is replaceable. It means that basicity of an acid depends only on the reaction and not on the number of hydrogen atoms in a chemical formula.