Answer
The pH of an $Na_2HPO_4$ solution will be greater than 7.
This happens because, this salt is formed by a insignificant acid $(Na^+)$ and a weak base** $(HP{O_4}^-)$, therefore, the solution is basic.
** This compound is amphiprotic, but, since its $K_b$ value is larger than its $K_a$ value, it will act more like a base.
Work Step by Step
To predict the pH range of an salt:
1. Identify the ions that form this salt, and compare its strengths:
2 $Na^+$: Conjugate acid of a strong base $(NaOH)$: Insignificant acid.
1 ${HPO_4}^{2-}$: Amphiprotic compound, but it is more basic than it is acidic $(K_b > K_a)$
Therefore: ${HPO_4}^{2-}$ is the stronger electrolyte, so, the solution is basic.
** You can find the constants values on page 624, table 14.2.
