In balancing the reaction N2H4(g) + ClO3−(aq) → NO(g) + Cl−(aq) in basic solution, what is the smallest possible integer coefficient in front of ClO3−?

• A. 2
• B. 3
• C. 4
• D. 1

Answer: (C)

Balancing redox reactions by tracking electrons is the key idea here. Each chlorate ion reduced to chloride gains 6 electrons. Each hydrazine molecule oxidized to nitrogen monoxide leaves behind 8 electrons (because both nitrogen atoms go from −2 to +2). To have the electrons lost equal to electrons gained, the amounts must satisfy 8a = 6b, where a is the number of hydrazine molecules and b the number of chlorate ions. The smallest whole-number solution is a = 3 and b = 4. So the smallest coefficient in front of chlorate is 4. In the full balanced equation, this corresponds to three hydrazine reacting with four chlorate to yield six NO and four Cl−, with water and hydroxide balancing the hydrogen and oxygen in basic solution.