Question

For ${{H}_{3}}P{{O}_{3}}$and ${{H}_{3}}P{{O}_{4}}$ the correct choice is:
A. ${{H}_{3}}P{{O}_{3}}$ is dibasic and reducing.
B. ${{H}_{3}}P{{O}_{3}}$ is dibasic and non-reducing.
C. ${{H}_{3}}P{{O}_{4}}$ is tribasic and reducing.
D. ${{H}_{3}}P{{O}_{4}}$ is tribasic and non-reducing.

Answer 1

Hint: Phosphoric acids are strong acids. It has 3 hydrogen atoms but only 2 are isonisable. Thus it is soluble in water.

Complete step by step solution:
Phosphorus acid or phosphonic acid is a colourless deliquescent crystalline solid which is highly acidic in nature. Even though it has 3 hydrogen, only 2 are ionisable. Hence it is dibasic acid and ionises as
${{H}_{3}}P{{O}_{3}}\to {{H}^{+}}+{{H}_{2}}PO_{3}^{-}$
${{H}_{3}}PO_{3}^{-}\to {{H}^{+}}+{{H}_{2}}PO_{3}^{2-}$
The other hydrogen, which does not take part in ionisation, has a reducing nature. Thus ${{H}_{3}}P{{O}_{3}}$ act as a reducing agent. So the correct answer for the question is (a).
-Phosphorus acid is obtained by hydrolysis of phosphorus trichloride and phosphorus trioxide.
-Phosphorus acid and its salts are strong reducing agents, as they are readily oxidisable to phosphoric acid and phosphates, respectively.
$HPO_{3}^{2-}+3O{{H}^{-}}\to PO_{4}^{3-}+2{{H}_{2}}O+2{{e}^{-}}$
- Phosphorus acid has the capability of reducing salts of copper, silver, gold etc., to their respective metals.
-The structure of phosphorus acid is shown as

-Orthophosphoric acid ${{H}_{3}}P{{O}_{4}}$ is a tribasic weak acid. It also has 3 hydrogen atoms, but has 4 oxygen atoms. It is not a reducing agent because all the 3 of its hydrogen ionises in water. It structure is shown as

-Phosphoric acid is a syrup liquid which combines with water gives a semi hydrate $2{{H}_{3}}P{{O}_{4}}.{{H}_{2}}O$
-It ionises in 3 stages
${{H}_{3}}PO_{4}^{{}}\rightleftharpoons {{H}^{+}}+{{H}_{2}}PO_{4}^{-}$
${{H}_{3}}PO_{4}^{-}\rightleftharpoons {{H}^{+}}+{{H}_{2}}PO_{4}^{2-}$
${{H}_{3}}PO_{4}^{2-}\rightleftharpoons {{H}^{+}}+PO_{4}^{3-}$
Its first ionisation takes place quite fast, but the second and third ionisation takes place very slow. It forms a series of salts.
Thus, the correct answer is option (a), Phosphorus acid is dibasic and reducing.

Note: There are chances that we assume even phosphoric acid is reducing like phosphorus acid. But it is not a reducing agent.