# Will monosodium dihydrogen phosphate react as base or acid in water?

Will $$\ce{NaH2PO4}$$ react as as an acid or a base if you make it react with water?

I got stuck here. I can figure out the reaction:

$$\ce{NaH2PO4 + H2O -> Na^+ + H2PO4^- + H2O}$$

But when I look $$\ce{H2PO4^{-}}$$ up, it says that it can both be an acid and a base. Any ideas?

Monosodium dihydrogen phosphate, $$\ce{H2PO4^-}$$, is an amphoteric species and it will act as both an acid and a base. These are the related chemical equations:
\begin{align} \ce{H2PO4- + H2O &<=> HPO4^2- + H3O+} & \mathrm{p}K_\mathrm{a} &= 7.21\\ \ce{H2PO4- + H2O &<=> H3PO4 + OH-} & \mathrm{p}K_\mathrm{b} &= 11.88 \end{align}
As an example, $$\pu{0.1 M}$$ solution of such a salt will have a pH of about $$4.7$$. In other words, the acidic behaviour dominates over the alkaline one. This is due to the value of $$\mathrm{p}K_\mathrm{a}=7.21$$ (related to the acidic behaviour) compared to the $$\mathrm{p}K_\mathrm{b}=11.88$$ (related to the alkaline behaviour).