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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?

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1 Answer 1

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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).

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  • $\begingroup$ Wouldn't it be acidic on every concentration in water? $\endgroup$
    – FloriOn
    Jan 17, 2017 at 11:29
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    $\begingroup$ Acidic yes, but pH would vary with concentration although slightly. $\endgroup$
    – mannaia
    Jan 19, 2017 at 17:42
  • $\begingroup$ if acidic behavior dominates, why would it be amphoteric ? $\endgroup$ Nov 12, 2021 at 0:32

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