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HC2H3O2(aq) + H2O(l) → H3O⁺(aq) + C2H3O2⁻(aq)
↓ ↓ ↓ ↓
acid base acid base
If we consider the only forward reaction H3O⁺is the conjugate acid of the base H2O . For reversse reaction CH3COOH is the conjugate acid of the base CH3COO⁻.
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