Hello!
The pair which represents an acid and its conjugate base is HSO₄⁻ and SO₄⁻².
To know if a pair of substances represents an acid and its conjugate base, one should look at the chemical structure of each substance. The conjugate base of an acid has usually the same chemical formula than the acid, but with one less proton (H⁺). Of the pairs of substances on the list, only HSO₄⁻ and SO₄⁻² have similar structures, with the only difference being the number of protons. The chemical reaction of dissociation of HSO₄⁻ is the following:
HSO₄⁻ + H₂O ⇄ SO₄⁻² + H₃O⁺
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