In the balanced acidic equation for Cl2(g) + S2O3^2−(aq) → Cl−(aq) + SO4^2−(aq), what is the coefficient of SO4^2−?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

Balancing redox reactions in acidic solution uses the half-reaction method: separate oxidation and reduction, balance atoms and charge with water and protons, then cancel electrons.

In this reaction, thiosulfate is oxidized to sulfate and chlorine gas is reduced to chloride. For the oxidation half-reaction, balance sulfur first (two S atoms on each side). Then balance oxygen with water and hydrogen with H+, and finally balance charge with electrons. You get: S2O3^2- + 5 H2O → 2 SO4^2- + 10 H+ + 8 e-. This shows that eight electrons are produced.

For the reduction half-reaction, chlorine gas gains electrons: Cl2 + 2 e- → 2 Cl-. To match the eight electrons produced, multiply by four: 4 Cl2 + 8 e- → 8 Cl-.

Add the two half-reactions and cancel the electrons. The combined balanced equation becomes S2O3^2- + 5 H2O + 4 Cl2 → 2 SO4^2- + 10 H+ + 8 Cl-. The sulfate appears with coefficient 2, so the correct coefficient is 2.