Question

How many $P - OH$ bonds in pyrophosphoric acid are?
A.Two
B.Three
C.Four
D.One

Answer 1

Hint:This question gives the knowledge about the oxoacids of phosphorus. Basically, the acids which contain oxygen atoms are oxoacids. Pyrophosphoric acid is the oxoacids of phosphorus. It also gives knowledge about tetrabasic acids.

Complete answer:
The acids which contain oxygen atoms are oxoacids. Phosphorus forms various numbers of oxoacids. Some examples of oxoacids of phosphorus are phosphoric acid, pyrophosphoric acid, metaphosphoric acid, hypophosphorous acid, peroxophosphoric acid, orthophosphoric acid and so forth. In oxoacids of phosphorus only $P - OH$ bonds donate hydronium ions, $P - H$ bonds do not donate the hydronium ions. In the structures of oxoacids of phosphorus, the oxidation state of phosphorus is always less than $ + 5$.

Pyrophosphoric acid is the oxoacids of phosphorus. It is a strong inorganic acid. The molecular formula of pyrophosphoric acid is ${H_4}{P_2}{O_7}$. The molecular formula of pyrophosphoric acid is $177.973g/mol$. It is generally soluble in water. Pyrophosphoric acid is a tetrabasic acid which means it can donate four hydronium ions. Heating of orthophosphoric acid at ${250^ \circ }C$ leads to the formation of pyrophosphoric acid.

The reaction of pyrophoric acid is as follows:
$2{H_3}P{O_4} \to {H_4}{P_2}{O_7} + {H_2}O$
Pyrophosphoric acid is also called diphosphoric acid. It contains four $P - OH$ bonds which can donate their hydronium ions. It is a tetrabasic acid which means it can donate four hydronium ions to bases in various acid-base reactions.
Therefore, it contains four $P - OH$ bonds.

Hence, option $C$ is correct.

Note:
Always remember that only $P - OH$ bonds donate hydronium ions. $P - H$ bonds do not donate the hydronium ions. Pyrophosphoric acid is a tetrabasic acid which means it can donate four hydronium ions.