Co³⁺(aq) + Hg(l) → Co²⁺(aq) + Hg₂²⁺(aq)

acidic environment

[Co³⁺(aq) + e^– → Co²⁺(aq)] × 2

2 Hg(l) → Hg₂²⁺(aq) + 2 e^–

sum and eliminate electrons

2 Co³⁺(aq) + 2 Hg(l) → 2 Co²⁺(aq) + Hg₂²⁺(aq)