Which oxoacid of phosphorous has the highest number of oxygen atoms present in its chemical formula?

We need to find which oxoacid of phosphorus has the highest number of oxygen atoms in its chemical formula.

Writing the chemical formulae of each oxoacid yields the following:

Option A - Pyrophosphorous acid: $$H_4P_2O_5$$ — contains 5 oxygen atoms.

Option B - Hypophosphoric acid: $$H_4P_2O_6$$ — contains 6 oxygen atoms.

Option C - Phosphoric acid: $$H_3PO_4$$ — contains 4 oxygen atoms.

Option D - Pyrophosphoric acid: $$H_4P_2O_7$$ — contains 7 oxygen atoms.

Comparing the number of oxygen atoms confirms that Pyrophosphoric acid ($$H_4P_2O_7$$) has the highest number of oxygen atoms (7). It is formed by the condensation of two molecules of phosphoric acid with loss of one water molecule:

$$ 2H_3PO_4 \xrightarrow{\Delta} H_4P_2O_7 + H_2O $$

The correct answer is Option D: Pyrophosphoric acid.