Answer
Acetic acid and acetate ion are capable of making a buffer with $3.31 \times 10^{-6}M$ of $[H_3O^+]$.
This happens because the Ka value for acetic acid is close to this hydronium ion concentration value.
Work Step by Step
1. Analyze the $[H_3O^+](3.31 \times 10^{-6}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table 15-1 on page 659.
- Acetic acid and acetate ion: Ka = $1.8 \times 10^{-5}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $3.31 \times 10^{-6}$
