Na2HPO4 is amphoteric: write the two reactions $\ce{Na_2HPO_4}$ is amphoteric, which means it can act as a base or as a acid depending on which substance they react with.

This is the reaction in which disodium phosphate appears to be an acid:

$$\ce{Na_2HPO_4 + H_2O \rightarrow H3O+ + Na_2PO_4-}$$

This is when it acts as a base:

$$\ce{Na_2HPO_4 + H_2O \rightarrow OH- + Na_2H_2PO_4+}$$

Did I write those two reactions currently? If not, what's wrong with my reasoning?
$\ce{Na2HPO4}$ is dissociated in solution. The amphoteric species is actually the hydrogen phosphate dianion:

Reaction as an acid:
$$\ce{HPO4^{2-} + H2O &lt;=&gt; PO4^{3-} + H3O+}$$

Reaction as a base:
$$\ce{HPO4^{2-} + H2O &lt;=&gt; H2PO4- + OH-}$$