I have a homework question that says

Write an equation when $\ce{NaH2PO4}$ and $\ce{NH4Cl}$ are mixed.

I assume that this is an acid base neutralization and will probably also give a salt due to the presence of $\ce{Cl-}$ and $\ce{Na+}$. So I figured out that the products will be $\ce{NaCl + H3PO4 + NH3}$, but I am not sure whether I am correct.

  • $\begingroup$ Your conclusion sounds reasonable to me: the basic character of amphoteric $\ce{H2PO4-}$ is triggered by the acidic one of $\ce{NH4+}$. Of course, a chemical equilibrium is established. $\endgroup$ – mannaia Feb 14 '14 at 7:44
  • $\begingroup$ Mixing solids doesn't really make a lot of sense to me. If the compounds were mixed in solution then you'd really end up with a bunch of hydrated ions not compounds. $\endgroup$ – MaxW Oct 23 '15 at 7:34

Your finding is indeed correct. Developping my comment above, this is what happens in solution. The two salts fully dissociate in water

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

$\ce{NH4Cl ->NH4+ + Cl-}$

Mixing them yields the following acid-base chemical equlibrium:

$\ce{NH4+ + H2PO4- <=> NH3 + H3PO4 }$

If one mixes solutions of equal concentrations (e.g.: 0.1 M), the resulting pH will be acidic (~4.7 if 0.1 M for both), since $\ce{NH4+}$ is a stronger acid than what $\ce{H2PO4-}$ is as a base. Or, similarly, $\ce{H3PO4}$ is a stronger acid than what $\ce{NH3}$ is as a base.

One may argue that $\ce{H2PO4-}$ has also acidic properties, being an amphoteric species: that's true, but in presence of an acid ($\ce{NH4+}$), it will act as a base. $\ce{Na+}$ and $\ce{Cl-}$ dwell in solution as spectator ions and do not have any signifcant effect on the pH.

  • $\begingroup$ Isn't phosphoric acid much stronger than ammonium ion? I think there is no significant reaction at all. $\endgroup$ – Oscar Lanzi Aug 5 '17 at 22:39

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