Question

The anhydride of orthophosphoric acid is:
A.\[{{\text{P}}_4}{{\text{O}}_6}\]
B.\[{{\text{P}}_2}{{\text{O}}_4}\]
C.\[{{\text{P}}_4}{{\text{O}}_{10}}\]
D.\[{{\text{H}}_2}{{\text{P}}_2}{{\text{O}}_6}\]

Answer 1

Hint: Anhydride is formed by removal of water from the molecule. Remove the three water molecules from orthophosphoric acid and we will get the anhydride.

Complete step by step answer:
The molecular formula for phosphoric acid is \[{{\text{H}}_3}{\text{P}}{{\text{O}}_4}\]. In the formation of anhydride, it loses all the three acidic hydrogen as water. The reaction occurs as:
\[{\text{2}}{{\text{H}}_3}{\text{P}}{{\text{O}}_4} \to {{\text{P}}_2}{{\text{O}}_5} + 3{{\text{H}}_2}{\text{O}}\]
The \[{{\text{P}}_2}{{\text{O}}_5}\] dimerize to form \[{{\text{P}}_4}{{\text{O}}_{10}}\].

Hence the correct option is C.

Additional information:
Orthophosphoric acid is also known as phosphoric acid. It is a weak acid. The chemical formula for phosphoric acid is \[{{\text{H}}_3}{\text{P}}{{\text{O}}_4}\]. All the three hydrogen are acidic in nature. That means when it is a triprotic acid, because it gives 3 hydrogen ions or protons upon dissociation. The formation of orthophosphate ions occurs when all the three hydrogen gets dissociated. It is colourless solid in pure state but we usually see the aqueous solution of phosphoric acid in laboratories. It is a weak acid and hence the value of dissociation constant is less in this case. The major amount of production of phosphoric acid is used as a fertilizer. About ninety percent of the produced amount is used as fertilizer. Other than this it is used as an additive, in fuel cells, in semiconductors and pH adjuster and so on.

Note:
Phosphoric acid is used as an additive in food especially in soft drinks. This is used to acidify the soft drinks to provide the sour taste. The intake of phosphoric acid can cause erosion in teeth and it also contributes to the formation of kidney stones.