Question 6

Let's examine the structures of the phosphorus oxoacids: (A) $H_3PO_4$ (Phosphoric acid): The structure is $O=P(OH)_3$. It contains only $P=O$ and $P-OH$ bonds. No $P-H$ bonds. (B) $H_3PO_3$ (Phosphorous acid): It exists in a tautomeric form $O=PH(OH)_2$. It contains one $P-H$ bond. (C) $H_4P_2O_7$ (Pyrophosphoric acid): The structure is $(HO)_2P(=O)-O-P(=O)(OH)_2$. It contains $P=O$, $P-OH$, and $P-O-P$ bonds. No $P-H$ bonds. (D) $H_3PO_2$ (Hypophosphorous acid): The structure is $O=PH_2(OH)$. It contains two $P-H$ bonds. Therefore, $H_3PO_3$ and $H_3PO_2$ contain $P-H$ bonds.