Question

Phosphoric acid ${{H}_{3}}P{{O}_{4}}$ cannot be neutralized to
(a) $HPO_{4}^{2-}$
(b) $PO_{4}^{3-}$
(c) ${{H}_{2}}PO_{4}^{-}$
(d) $HPO_{3}^{2-}$

Answer 1

Hint: Phosphoric acid is a weak acid and consists of only three ionizable hydrogen atoms due to the presence of three \[P-OH\]bonds and neither of the oxygen atoms is ionized in the phosphoric acid. Now, by this you can easily identify the anion into which the phosphoric acid can be neutralized. Solve it.

Complete answer:
First of all, let’s discuss the phosphoric acid. Phosphoric acid is an inorganic solid compound and has the chemical formula as ${{H}_{3}}P{{O}_{4}}$ and is made up of three atoms i.e. hydrogen, phosphorus and oxygen atoms .
Phosphoric acid is commonly also known as the orthophosphoric acid and is weakly acidic in nature and is colorless. It is sparingly soluble in water but completely soluble in alcohol. On the other hand, by neutralization we mean that chemical reactions in which acid and base neutralize each other and remove the excess of hydrogen and the oxygen atoms.
Now considering the statement;
Phosphoric acid consists of three \[P-OH\] bonds and one \[P=O\] bond. Since, it has three \[P-OH\]bonds, so it has three ionizable hydrogen atoms and therefore, it behaves as tribasic and triprotic and ionizes in three stages as;
\[\begin{align}
& {{H}_{3}}P{{O}_{4}}\rightleftharpoons {{H}^{+}}+{{H}_{2}}PO_{4}^{-} \\
& {{H}_{2}}PO_{4}^{-}\rightleftharpoons {{H}^{+}}+HPO_{4}^{2-} \\
& HPO_{4}^{2-}\rightleftharpoons P{{O}_{4}} \\
\end{align}\]
Hence, the phosphoric acid can be neutralized to $HPO_{4}^{2-}$, $PO_{4}^{3-}$ and \[{{H}_{2}}PO_{4}^{-}\] and cannot be neutralized to $HPO_{3}^{2-}$.

So, option (d) is correct.

Note: Due to its tribasic or triprotic nature, phosphoric acid exists in three series of the salts such as sodium dihydrogen phosphate i.e. $Na{{H}_{2}}P{{O}_{4}}$, disodium hydrogen phosphate i.e. $N{{a}_{2}}HP{{O}_{4}}$ and sodium phosphate i.e. $N{{a}_{3}}P{{O}_{4}}$.