Answer
$HC{O_3}^-$ and $C{O_3}^{2-}$ are capable of making a buffer with $9.7 \times 10^{-11}M$ of $H_3O^+$
This happens because the $K_a$ value for $HC{O_3}^-$ is close to this hydronium ion concentration value.
Work Step by Step
1. Analyze the $[H_3O^+] (9.7 \times 10^{-11}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table $15-1$ on page $659$.
- $HC{O_3}^-$ and $C{O_3}^{2-}$: $K_a$ = $4.7 \times 10^{-11}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $9.7 \times 10^{-11}M.$
