Question

Give two examples of tribasic acid.

Answer 1

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.

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.