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Why is ammonium such a weak acid $(\mathrm pK_\mathrm a = 9.24)$, if $\ce{NH4+}$ has a positive charge on nitrogen, while $\ce{NH3}$ is neutral. It means that conjugate base (ammonia) is really stable, which makes conjugate acid a strong acid.

Another argument why ammonium should be strong acid is comparison with hydronium ion $\ce{H3O+}$. Both nitrogen (EN = 3.04) and oxygen (EN = 3.44) are significantly electronegative atoms and "don't like" to be positively charged. That explains pretty good hydronium ion $\mathrm pK_\mathrm a = -1.74$, but why is then ammonium $\mathrm pK_\mathrm a$ is just $9.24$?

nons3rviam
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  • Ammonia is rather apolar (compared to water!). So apolar its Bp. is -33°C. It's really only soluble in water because it can take up a proton. – Karl Feb 05 '17 at 14:56
  • Check your acid-base chemistry: A very weak base is conjugate to a strong acid. – Karl Feb 05 '17 at 15:11

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As chemists we are given experimental observations, such as the difference in measured pKa between ammonium and hydronium ions, and we invent concepts such as "electronegativity" to explain this result in conjunction with many other similar results. If this concept is insufficient in your mind to explain the experimental data, you may be motivated to seek a more satisfying and self-consistent explanation. For me electronegativity is sufficient in this case. The fact that ammonia is "stable" and can be put in a bottle does not mean that it is somehow unstable when protonated to give ammonium ion. Many ammonium salts can be put in a bottle, even salts of weak acids like carbonic. Having a positive charge on an atom is not in itself an unstable state-note how stable salts like sodium chloride (with a full positive charge on sodium) are, as well as species like tetramethylammonium ion-- you can make the hydroxide and put it in a bottle.

iad22agp
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