Question

A tribasic acid with peroxy bond is:
(a)${{H}_{3}}P{{O}_{2}}$
(b) ${{H}_{3}}P{{O}_{3}}$
(c) ${{H}_{3}}P{{O}_{4}}$
(d) ${{H}_{3}}P{{O}_{5}}$

Answer 1

A tribasic acid means an acid which could donate three ${{H}^{+}}$ , this implies there will be 3 hydroxide groups in the molecule. A peroxy bond is a bond between two oxygen atoms (generally of the type A-O-O-H), thus the molecule with this type of bond will be the tribasic acid with peroxy linkage.

Complete step-by-step answer:
First we will discuss the method of calculation of peroxy linkages:-
- Since all the tribasic acid given above have Phosphorus as the central atom therefore we will consider its maximum oxidation state which is 5 (maximum oxidation state is equivalent to number of valence electrons).
- Then we will calculate the oxidation state of the phosphorus atom in each compound using the X-Method.
- If the oxidation state calculated through X-Method is greater than maximum oxidation state of the same atom, then that acid contains peroxy linkage.
- Number of peroxy linkages can be calculated as follows:-
(A) If number of central atom =1, then number of peroxy linkage = $\dfrac{\text{Calculated oxidation state by X-Method - Maximum oxidation state}}{2}$
(B) If number of central atom =2, then number of peroxy linkage = $\dfrac{\text{Calculated oxidation state by X-Method - Maximum oxidation state}}{1}$
-If 2 or more central atoms are present then peroxy linkage is represented as: A-O-O-A and if only 1 central atom is present then peroxy linkage is represented as: A-O-O-H.

Calculation of peroxy linkages:-
(a)${{H}_{3}}P{{O}_{2}}$
Maximum oxidation state of P: 5
Calculated oxidation state of P: 3(1) + x +2(-2) = 0
x = 1
Since maximum oxidation state is greater than calculated oxidation state, ${{H}_{3}}P{{O}_{2}}$ do not have any peroxy linkage.

(b) ${{H}_{3}}P{{O}_{3}}$
Maximum oxidation state of P: 5
Calculated oxidation state of P: 3(1) + x +3(-2) = 0
x = 3
Since maximum oxidation state is greater than calculated oxidation state, ${{H}_{3}}P{{O}_{3}}$ do not have any peroxy linkage.

(c) ${{H}_{3}}P{{O}_{4}}$
Maximum oxidation state of P: 5
Calculated oxidation state of P: 3(1) + x +4(-2) = 0
x = 5
Since maximum oxidation state is equal to calculated oxidation state, ${{H}_{3}}P{{O}_{4}}$ do not have any peroxy linkage.

(d) ${{H}_{3}}P{{O}_{5}}$
Maximum oxidation state of P: 5
Calculated oxidation state of P: 3(1) + x +5(-2) = 0
x = 7
Since calculated oxidation state is greater than maximum oxidation state, ${{H}_{3}}P{{O}_{5}}$ do have peroxy linkage.
-Calculation of number of peroxy linkages in${{H}_{3}}P{{O}_{5}}$:-
Since the number of central atom (P) =1, then number of peroxy linkage in ${{H}_{3}}P{{O}_{5}}$ = $\dfrac{\text{Calculated oxidation state by X-Method - Maximum oxidation state}}{2}$
$\dfrac{7-5}{2}=1\text{ peroxy linkage}$
The structure of ${{H}_{3}}P{{O}_{5}}$is shown below:-

Therefore, the tribasic acid with peroxy bond is: (d) ${{H}_{3}}P{{O}_{5}}$.

Note: -Always while drawing structures of oxyacids of Phosphorus, first draw a P=O bond and then make the rest of the structure. (As each oxy acid of Phosphorus contains a P=O bond).
-Peroxy acids contain A-O-O-H bonds and thus are generally strong oxidizers. They can oxidize alkenes to epoxides and ketones to esters.