Question

Incorrect statement about pyrophosphorous acid ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ is:
(A) It contains P in +5 oxidation state.
(B) It is dibasic.
(C) It is strongly reduced in nature.
(D) It contains one $P-O-P$ bond.

Answer 1

Pyrophosphorous acid ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ is an oxoacid of phosphorus. All the oxoacids of phosphorus have at least one $P=O$ and one $P-OH$ bond in their structure.

Complete step by step answer:
In oxoacids, phosphorus have four groups or atoms arranged in a tetrahedron. Structure of pyrophosphorous acid ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ is given below:

Some of the important characteristics of pyrophosphorous acid ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ are discussed below:
It is clear from the structure given above that ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ contains two $P=O$, two $P-H$, two $P-OH$ and one $P-O-P$ bond.
Oxidation state of P in ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ can be calculated as follows:
Let the oxidation state of P be ‘x’. Oxidation state of H is +1 and of O is taken as -2.
$4\times 1+2x+5\times (-2)=0$
$4+2x-10=0$
$2x-6=0$
$x=3$
Therefore, P is present in +3 oxidation state.
The $O-H$ bonds present in ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ are ionizable and give ${{H}^{+}}$ ions. Basicity of oxoacids depends on the number of ${{H}^{+}}$ ions. Thus, the basicity of ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ is 2 i.e. it is a dibasic acid.
Greater the number of $P-H$ bonds, stronger is the reducing agent. ${{H}_{4}}{{P}_{2}}{{O}_{5}}$ contains two $P-H$ bonds thus, it is strongly reduced in nature.
Based on the above discussed points, it is clear that the oxidation state of phosphorus in pyrophosphorous acid is +3 and not +5.

So, the correct answer to the question is option (A).

Note: Carefully calculate the oxidation state and the number of different bonds present. Do not confuse pyrophosphorous acid with any other oxoacid of phosphorus.