I know the common strong and weak acids but if a mineral acid like $\ce{H3BO3}$ springs up, I can't actually tell whether it is strong or weak. Also can I justify in chemical terms why $\ce{H2SO4}$ is a strong acid but $\ce{H2SO3}$ a weak one?
Why is $\ce{HNO3}$ a strong acid whereas $\ce{HNO2}$ a weak one ?


The above link shares a list of strong and weak acids but fails to state why one acid is weak while the other strong.

  • $\begingroup$ chemistrytextbookcrawl.blogspot.com/2013/02/paulings-rules.html Apart from that 8-5p formula itself, you obviously have to know the molecular structure of the acid, i.e. you will have to know that H3BO3 is actually B(OH)3 which means that the approximate pKa as given by Pauling's rules is 8 - 5(0) = 8. On the other hand H3PO2, H3PO3 and H3PO4 all have very similar pKa's because p = 1 for all three. $\endgroup$ – orthocresol Aug 18 '16 at 7:21

Is there a formula or way to determine whether an acid is a strong one or a weak one without conducting experiments in real?

Yes, there are ways to determine whether an acid is strong or weak without going into the lab yourself. That being said, someone must still go into the lab.

The most straightforward way would be calculating ΔG for an acid-base reaction. Remember, a negative delta G means that a reaction is spontaneous. Since strong acids have a Ka > 1, their ΔG values should be negative.


A "strong" acid simply means that when added to water, the acid will dissociate completely, therefore a strong acid is strong because it dissociates completely in aqueous solution while a weak acid is weak because it does not dissociate completely in aqueous solution. There are only 7 common inorganic strong acids so I would simply memorize them.

As far as why they are strong acids i.e. why they dissociate completely while other acids do not is a little more complicated and to be honest I am probably not the person to ask. I haven't looked into inorganic acids too extensively, though organic acids you must look at the stability of the conjugate base. A more stable conjugate base means the acid will more easily donate a proton. I do know for a fact that the conjugate base of sulfuric acid exhibits resonance which is a major stabilizing factor. However, based on the structure of sulfurous acid I would expect it to also exhibit resonance, though I could be wrong.

The acid dissociation constant Ka is determined experimentally. Strong acids have a Ka that is >> 1 while weak acids have a Ka that is << 1.

Hopefully I answered your question, but if not I'm sure someone with a little more inorganic chemistry experience will fill in the holes.

  • $\begingroup$ That there's only 7 common strong acids mentioned in a textbook or somewhere doesn't mean that these are only ones; in the future put in answers things you're really sure, best if you have sources to back it up. $\endgroup$ – Mithoron Aug 18 '16 at 11:04
  • $\begingroup$ @Mithoron Sorry, that was what I was taught throughout college, and I even had re-googled it, finding several college websites that had lists of "the 7 strong acids". I was unaware there were more. But seeing as I am in my 4th year at an acredited university working towards a BS in Biochemistry and all they've expected me to know are 7 of them (HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3), Im sure this is all the asker will be expected to know. I figured a grad student wouldn't be asking the definition of a strong acid. My apologies though, I am far more interested in organic chemistry. $\endgroup$ – KeatonB Aug 18 '16 at 16:41
  • $\begingroup$ @Mithoron Correct me if I'm wrong but I do believe there are only 7 strong inorganic acids which I believe is all the asker was referring to. I actually did know there were more strong acids, but they are all organic acids. I have edited my post to omit the ambiguity. $\endgroup$ – KeatonB Aug 18 '16 at 16:59
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    $\begingroup$ I meant strong inorganic acids - it's impossible to tell how many of them exists - there may be millions of inorganic compounds and lots of them are strong acids, starting from H2SeO4 through halogenated carboranes to various complexes. $\endgroup$ – Mithoron Aug 18 '16 at 18:25
  • $\begingroup$ +1 to @Mithoron's comment. Tetrafluoroboric acid is also quite strong. But, it is less common, and I think the problem is that the list of 7 is really the list of 7 most common inorganic acids (that one might encounter in an intro chem class). $\endgroup$ – Zhe Nov 3 '16 at 17:34

Short answer: In general, no; but oxyacids and polyprotic acids have an extensive literature. (See Wikipedia: Acid dissociation constant). Your second question, which is a "why" question is best answered on a case-by-case basis. Your question implies to me that you are not familiar with the quantum mechanical aspects of the dissociation of a protic acid in aqueous solution. I'm pretty sure they would almost immediately require much more advanced knowledge of quantum mechanics (specifically, DFT (and if you don't know what DFT is, then that proves my point)) than you have. In general, modern Physics and Chemistry (the hard physical sciences) aren't so good at answering "why?" questions. I encourage you to read (at least twice) the Wikipedia article Acid dissociation constant. It mentions a variety of contributing factors which influence dissociation. You have not only electrostatic effects (which in all but the most simple cases are quantum mechanical in origin (meaning require a QM explanation for all but the least sophisticated student - the least sophisticated students will accept half-baked explanations (like Pauling's 2 rules)) but steric effects, resonance effects, etc. QM is the basis for chemistry. Add statistical mechanics to that, and a "sufficiently powerful" computer, and you could recreate modern chemistry. By which I mean that fundamental "why?" questions can "really" only be addressed by QM. I'm thinking that that is not what you're looking for. (but, there it is.)


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