Question

Give two examples of tribasic acid.

Answer 1

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.

Note:
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.