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At low concentrations of aqueous $\ce{NH4HF2}$ $(w = 1–2\,\%)$ $\mathrm{pH}\approx 3.$

But aqueous $\ce{NH4F}$ $(w = 20–40\,\%)$ has $\mathrm{pH}\approx 7.$

Yet the equilibria of

$$ \begin{align} \ce{HF &-> H+ + F-} &\quad K &= \pu{6.85E-6 M} \tag{1} \\ \ce{HF + F- &-> HF2-} &\quad K &= \pu{3.963 M^-1} \tag{2} \end{align} $$

I understand that $\ce{NH4F}$ comes from a weak acid and a weak base. So, the $\mathrm{pH}$ is around neutral. I don't understand how this equilibria makes $\ce{NH4HF2}$ more acidic. Does the second equilibria make the $\ce{HF2-}$ a stronger acid? I'm having a hard time understanding this.

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This question and the one referenced by Mithoron address $\ce{NH4HF2}$ as an entity in itself which must be examined as a whole.

It is easier conceptually to rewrite the formula for the compound as $\ce{NH4F·HF}$. Consider adding $\ce{NH4F}$ to water: you get a $\mathrm{pH}$ near $7.$ In a separate container, add $\ce{HF}$ to water (approx. $\pu{0.1 M});$ you get a $\mathrm{pH}\approx 1$.

Combine the solutions and the $\mathrm{pH}$ will settle out somewhere between $1$ and $7,$ because of the common ion effect: $\ce{F-}$ from the $\ce{NH4F}$ inhibits the ionization of $\ce{HF},$ but the solution is still acidic. The approximate $\mathrm{pH}$ can be calculated from the ionization constant of $\ce{HF}$ and the concentrations of $\ce{NH4F}$ and $\ce{HF}.$

The concept of the $\ce{HF2-}$ moiety dominating the discussion for a solution is probably making the analysis murkier than it needs to be. It's just a hydrogen-bonded complex that happens to retain some semblance of existence in the solid. It is fairly stable: mp $\pu{126 °C},$ bp $\pu{240 °C}$ (with decomposition).

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    $\begingroup$ "126$^0$ C" reads as "one hundred twenty-six to the power of zero coulombs". "NH$_4$F.HF" has a dot that doesn't make sense and the formula breaks across lines incorrectly. Please consider using proper formatting tools (mhchem, Unicode, plain MathJax) so that your notations make sense. $\endgroup$
    – andselisk
    Sep 27 '21 at 21:29
  • $\begingroup$ @andselisk: Well, thanks, I guess. It's not clear to me what straightforward notation, according to my screen, gets retranslated to a different format. I have already been notified about certain subscripts, but not the temperature. Is there a way that I can see the wrong view before I post it? $\endgroup$ Sep 28 '21 at 14:31
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    $\begingroup$ Let's try this: I corrected the answer a bit, and once you have some spare time, feel free to review the edits, maybe also by (re)visiting this page, this page and this one on how to format your future posts better with MathJax and Markdown. There are no other tools other than the preview windows, standardized notations (which everyone is supposed to look up) and common sense, I'm afraid. $\endgroup$
    – andselisk
    Sep 28 '21 at 15:55

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