Answer
Lactic acid and lactate ion are capable of making a buffer with $4.5 \times 10^{-3}M$ of $[H_3O^+]$.
This happens because the Ka value for lactic acid is close to this hydronium ion concentration value.
Work Step by Step
1. Analyze the $[H_3O^+](4.5 \times 10^{-3}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table 15−1 on page 659.
- Lactic acid and lactate ion: Ka = $1.4 \times 10^{-4}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $4.5 \times 10^{-3}$
