Answer
$H_2P{O_4}^-$ and $HP{O_4}^{2-}$ are capable of making a buffer with $5.2 \times 10^{-8}M$ of $H_3O^+$.
This happens because the $K_a$ value for $H_2P{O_4}^-$ is very close to this hydronium ion concentration value.
** You can also use $HClO$ and $ClO^-$, but $H_2P{O_4}^-$ and $HP{O_4}^{2-}$ are more suitable.
Work Step by Step
1. Analyze the $[H_3O^+] (5.2 \times 10^{-8}M)$, and find the conjugate acid-base pair with the closest $K_a$ value.
- Use Table $15-1$ on page $659$.
- $H_2P{O_4}^-$ and $HP{O_4}^{2-}$: $K_a$ = $6.3 \times 10^{-8}$
Therefore, this is the best pair that can be used to make a buffer with hydronium ion concentration equal to $5.2 \times 10^{-8}$
** $HClO$ and $ClO^-$ are capable of making this buffer too, but $H_2P{O_4}^-$ and $HP{O_4}^{2-}$ have a closer $K_a$ value.
