Question

The pair in which phosphorous atoms have a formal oxidation state of + 3 is :-

• A. Pyrophosphorous and pyrophosphoric acids
• B. Orthophosphorous and pyrophosphorous acids
• C. Pyrophosphorous and hypophosphoric acids
• D. Orthophosphorous and hypophosphoric acids

Answer 1

Answer: (B)

Orthophosphorous and pyrophosphorous acids

Answer 2

To determine the formal oxidation state of phosphorus in each compound, we use the common oxidation states of hydrogen (+1) and oxygen (-2). The sum of oxidation states in a neutral molecule is zero.

1. Pyrophosphorous acid (H₄P₂O₅):
   Let the oxidation state of P be x.
   $4(+1) + 2(x) + 5(-2) = 0$
   $4 + 2x - 10 = 0$
   $2x - 6 = 0$
   $2x = 6$
   $x = +3$

2. Pyrophosphoric acid (H₄P₂O₇):
   Let the oxidation state of P be x.
   $4(+1) + 2(x) + 7(-2) = 0$
   $4 + 2x - 14 = 0$
   $2x - 10 = 0$
   $2x = 10$
   $x = +5$

3. Orthophosphorous acid (H₃PO₃):
   Let the oxidation state of P be x.
   $3(+1) + x + 3(-2) = 0$
   $3 + x - 6 = 0$
   $x - 3 = 0$
   $x = +3$

4. Hypophosphoric acid (H₄P₂O₆):
   Let the oxidation state of P be x.
   $4(+1) + 2(x) + 6(-2) = 0$
   $4 + 2x - 12 = 0$
   $2x - 8 = 0$
   $2x = 8$
   $x = +4$

Now, let's check the given options:

• (1) Pyrophosphorous (+3) and pyrophosphoric (+5) acids: Not both +3.
• (2) Orthophosphorous (+3) and pyrophosphorous (+3) acids: Both have phosphorus in a +3 oxidation state. This is the correct pair.
• (3) Pyrophosphorous (+3) and hypophosphoric (+4) acids: Not both +3.
• (4) Orthophosphorous (+3) and hypophosphoric (+4) acids: Not both +3.