Answer:
Δ oxidation state of Arsenic per mole As = 2 electrons lost by oxidation.
Explanation:
Reaction as given should be written as...
2HNO₃ + 3H₃AsO₃ => 2NO + 3HAsO₃ + 4H₂O ... (Need 4H₂O to balance in acidic medium)
As in H₃AsO₃ = +3
As in HAsO₃ = +5
As⁺³ => As⁺⁵ + 2e⁻
∴ Δ oxidation state = 2 electrons lost by oxidation.
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Overall Rxn has 6e⁻ gained by reduction process along with 6e⁻ lost by oxidation process...
2HNO₃ + 2(3e⁻) => 2NO + 4H₂O
3H₃AsO₃ => 3HAsO₃ + 3(2e⁻)
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2HNO₃ + 3H₃AsO₃ => 2NO + 3HAsO₃ + 4H₂O
