Question

The number of $P - OH$ bonds and the oxidation state of phosphorus atom in pyrophosphoric acid (${H_4}{P_2}{O_7}$ ) respectively are:
A. Four and five
B. Four and four
C. Five and five
D. Five and four

Answer 1

Hint: Pyrophosphoric acid is an inorganic compound with the chemical formula ${H_4}{P_2}{O_7}$ . It is an odorless and colorless compound and is soluble in numerous solvents such as water, diethyl ether and ethyl alcohol. It is an anhydrous acid and crystallizes in two polymorphic forms.

Complete step by step answer:

The structure of pyrophosphoric acid is as follows:

As we can clearly see in the structure, the number of $P - OH$ bonds is 4.
Now let us determine the oxidation state of phosphorus atoms in the above structure of pyrophosphoric acid. The oxygen is an electronegative element and will always carry a negative charge. Now, it depends on its surrounding atoms about what charge it carries. In case of a hydroxyl group, the hydrogen carries a +1 charge. So, the opposite charge generated on the oxygen atom along with magnitude will be -1. In the case of only oxygen atoms, we know that the oxidation number of oxygen is equal to -2. So, the charge generated on the adjacent atom of oxygen will be +2. Thus, the oxidation state of phosphorus atom will be generated because of:
Oxidation state on phosphorous = $[2 \times (P - OH) + (1 \times = O) + (1 \times - O)]$ oxidation states
Thus, the oxidation state of both the phosphorus atoms will be:
${P_{O.S}} = 2 \times ( + 1) + 1 \times ( + 2) + 1 \times ( + 1) = + 5$
Thus, the correct option is A. Four and five.

Note:
Pyrophosphoric acid acts as a source of producing phosphoric acid. Since it is an anhydrous acid, on hydrolysis of one mole of pyrophosphoric acid, two moles of phosphoric acid is obtained. The reaction takes place as follows:
${H_4}{P_2}{O_7} + {H_2}O \to 2{H_3}P{O_4}$