Balancing redox reaction via oxidation state change

Steps:

• Nitrogen in NH3 has oxidation state -3; increases by 5 to +2, so product is NO.
• Unbalanced equation: NH3 + O2 → NO + H2O.
• For 4 mol NH3, 4 N atoms each lose 5 electrons, totaling 20 electrons transferred.
• Each O2 molecule gains 4 electrons (reduced to 2 O^{2-}), so 20/4 = 5 mol O2 required.

Why B is correct:

• 5 mol O2 exactly balances the 20 electrons lost by 4 N atoms, per redox electron transfer rule.

Why the others are wrong:

• A: 4 mol O2 gains only 16 electrons, insufficient for the 20 electrons lost.
• C: 7 mol O2 gains 28 electrons, exceeding the 20 electrons available.
• D: 10 mol O2 gains 40 electrons, far exceeding the 20 electrons transferred.

Final answer: B