H3O+ (hydronium ion) and OH– (hydroxide ion) are not a conjugate acid-base pair. To understand why, let’s first define what a conjugate acid-base pair is.
A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton (H+). The conjugate acid is formed when a base gains a proton, and the conjugate base is formed when an acid loses a proton.
In the case of H3O+ and OH–:
- H3O+ is the conjugate acid of H2O (water). When water gains a proton, it forms H3O+.
- OH– is the conjugate base of H2O. When water loses a proton, it forms OH–.
Since H3O+ and OH– are not directly related by the gain or loss of a single proton, they do not form a conjugate acid-base pair. Instead, they are part of separate conjugate pairs involving water.