Answer
Hydrogen carbonate ion and carbonate ion are capable of making a buffer with $7.08 \times 10^{-11}M$ of $[H_3O^+]$.
This happens because the Ka value for $HC{O_3}^-$ is close to this hydronium ion concentration value.
Work Step by Step
1. Analyze the $[H_3O^+](7.08 \times 10^{-11}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table 15-1 on page 659.
- Hydrogen carbonate ion and carbonate ion : Ka = $4.7 \times 10^{-11}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $7.08 \times 10^{-11}$.
