I was told to find the strongest and weakest acids out of $\ce{HCl, HClO, HClO2, HClO3}$, and $\ce{HClO4}$. I knew that $\ce{HClO}$ was the weakest but was stuck between $\ce{HCl}$ and $\ce{HClO4}$ for the strongest (as these are both strong acids). Is there a way to find the relative strength of strong acids without being given the $K_\mathrm{a}$ or $\mathrm{p}K_\mathrm{a}$? Or would I just have to have memorized that $\ce{HClO4}$ is generally stronger than $\ce{HCl}$?

  • $\begingroup$ As for your question, I don’t think there is any way to compare the two since there are no real analogies you can use. But I’m not sure enough to actually call this an answer. $\endgroup$ – Jan Nov 13 '15 at 19:59
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    $\begingroup$ Truthfully a lot of "facts" like this are asked in class tests/problems. In the real world if you use those chemicals then you just know, if you don't use them a lot you then you just look the data up. It is easy to rationalize HClO, HClO2, HClO3 and HClO4. You recognized HCl would be an odd-ball. sigh for now you'll just have to memorize it... $\endgroup$ – MaxW Nov 13 '15 at 20:07
  • $\begingroup$ Actually, some people would say that $\ce{HClO3}$ is a strong acid too. I don't think there's a real way to compare $\ce{HCl}$ and $\ce{HClO4}$. There are some empirical rules for estimating the $\mathrm{p}K_\mathrm{a}$ of an oxoacid: en.wikipedia.org/wiki/… but clearly they do not apply to $\ce{HCl}$. $\endgroup$ – orthocresol Nov 13 '15 at 20:28
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    $\begingroup$ Related: How to determine which substance is the strongest base/acid? $\endgroup$ – Loong Nov 13 '15 at 20:46