Final answer:
H2O / OH- is not an acid-base conjugate pair because H2O is a base that can accept a proton to become H3O+ (the conjugate acid), and OH- is the conjugate base that results from H2O losing a proton.
Step-by-step explanation:
The question at hand is which of the following is not an acid-base conjugate pair:
- HCl / Cl⁻
- H₂SO₄ / HSO₄⁻
- NH₄⁹ / NH₃
- H₂O / OH⁻
An acid-base conjugate pair consists of two species that transform into each other by the gain or loss of a proton (H⁹). For example, when HCl donates a proton (H⁹), it becomes Cl⁻, making HCl / Cl⁻ a conjugate acid-base pair. Similarly, H₂SO₄ can donate a proton to become HSO₄⁻, and NH₄⁹ can give up a proton to revert to NH₃. However, in the option presented as H₂O / OH⁻, H₂O is a base that accepts a proton to become H₃O⁹ (the conjugate acid), and OH⁻ is the conjugate base that results from H₂O losing a proton. Therefore, H₂O / OH⁻ ⁺not a conjugate acid-base pair; instead, H₂O and H₃O⁹ form such a pair.