Answer
Ammonium ion and ammonia are capable of making a buffer with $4.79 \times 10^{-9}M$ of $[H_3O^+]$.
This happens because the Ka value for ammonium ion is close to this hydronium ion concentration value.
Work Step by Step
1. Analyze the $[H_3O^+](4.79 \times 10^{-9}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table 15-1 on page 659.
- Ammonium ion and ammonia: Ka = $5.6 \times 10^{-10}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $4.79 \times 10^{-9}M.$
