Question

The oxidation states of 'P' in $H_4P_2O_7$, $H_4P_2O_5$ and $H_4P_2O_6$, respectively, are :

• A. 6, 4 and 5
• B. 5, 4 and 3
• C. 5, 3 and 4
• D. 7, 5 and 6

Hint:

While the algebraic sum method works for most simple oxyacids, be cautious with acids containing peroxide links (-O-O-) or direct element-element bonds (like the P-P bond in $H_4P_2O_6$). In those cases, determining the O.S. from the structure is the most reliable method.

Answer 1

The Correct Option is C

We can determine the oxidation state (O.S.) of phosphorus (P) in each compound by assigning the standard oxidation states to oxygen (-2) and hydrogen (+1). Let the O.S. of P be x.
1. For $H_4P_2O_7$ (Pyrophosphoric acid):
The sum of oxidation states is zero.
$4 \times (+1) + 2 \times (x) + 7 \times (-2) = 0$
$4 + 2x - 14 = 0$
$2x = 10 \implies x = +5$.
2. For $H_4P_2O_5$ (Diphosphorous acid):
$4 \times (+1) + 2 \times (x) + 5 \times (-2) = 0$
$4 + 2x - 10 = 0$
$2x = 6 \implies x = +3$.
3. For $H_4P_2O_6$ (Hypophosphoric acid):
This acid contains a direct P-P bond. When using the algebraic method, it gives the average oxidation state.
$4 \times (+1) + 2 \times (x) + 6 \times (-2) = 0$
$4 + 2x - 12 = 0$
$2x = 8 \implies x = +4$.
(Drawing the structure (HO)$_2$(O)P-P(O)(OH)$_2$ and assigning electronegativities confirms the +4 state for each P atom).
The respective oxidation states are +5, +3, and +4. This corresponds to option (C).