Which of the following shows a correct acid/conjugate base pair?

Question

 A. H₃PO₄/H₂PO₄^-

 B. NO₃^-/NaNO₃

 C. H₂SO₄/SO₄^2-

 D. HCl/Cl₂

Answer 1

A. H₃PO₄/H₂PO₄^-

because the acid / conjugate base should differ by only 1 hydrogen, as is the case here. For HCl/Cl₂, it would have been correct if it said HCl/Cl-, but Cl₂ is not a conjugate base of HCl.