Answer
$H_3O^+$ and $OH^-$ aren't a conjugate acid-base pair.
Work Step by Step
- Fast method:
A conjugate acid-base pair has exactly one $H^+$ as a difference. So, this isn't a valid pair.
- Other method:
1. Identify the base:
- $OH^-$
2. Add a $H^+$ to it:
- $H_2O$
Compare it with the other substance:
$H_2O \ne H_3O^+$
- So, this isn't a conjugate acid-base pair.
