Answer
$Na_2HPO_4$ and $Na_3P{O_4}$ are not the best combination for a buffer solution with pH $\approx$ 7.
Work Step by Step
1. Identify the acid:
$HP{O_4}^{2-}$.
2. Calculate its pKa:
$pKa = -log(Ka)$
$pKa = -log( 4.6 \times 10^{- 13})$
$pKa = 12.34$
3. Compare this number to 7.
- It is not approximately 7, therefore, these compounds are not the best combination for a buffer with neutral pH.
