Taking into account the Brønsted-Lowry acid-base theory, a base/conjugate acid pair is H₂O/H₃O⁺.
The Brønsted-Lowry acid-base theory (or the Brønsted-Lowry theory) identifies acids and bases based on whether the species accepts or donates protons or H⁺.
According to this theory, acids are proton donors while bases are proton acceptors. That is, an acid is a species that donates an H⁺ proton while a base is a chemical species that accepts an H⁺ proton from the acid.
So, reactions between acids and bases are H⁺ proton transfer reactions, causing the acid to form its conjugate base and the base to form its conjugated acid by exchanging a proton.
In other words, a conjugate base is an ion or molecule resulting from the acid that loses the proton, while a conjugate acid is an ion or molecule resulting from the base that gains the proton:
acid + base ⇄ conjugate base + conjugate acid
In this case, you know:
H₂PO₄ + H₂O → H₃O⁺ + HPO₄⁻
H₂PO₄ behaves like acid because donates an H⁺ proton while H₂O behaves like base because accepts an H⁺ proton from the acid.
So, H₃O⁺ is the conjugate acid of the H₂O and HPO₄⁻ is conjugate base of H₂PO₄.
In summary, a base/conjugate acid pair is H₂O/H₃O⁺.
Learn more about the Brønsted-Lowry acid-base theory:
