Since the six strong acids dissociate completely into their ions, and the reaction goes to completion, does this mean that acids such as $\ce{HI}$ and $\ce{HCl}$ have the same acidic strength?

I know that 0.10 M $\ce{HI_{(aq)}}$ and 0.10 M $\ce{HCl_{(aq)}}$ both contain 0.10 M $\ce{H3O+_{(aq)}}$, which leads me to believe they are equally strong acids. Is this correct?


2 Answers 2


There's something called the "solvent leveling effect". In short, there's a lowest pKa in a particular solvent, based on the basicity of the conjugate base.

In water, you're limited by $\ce{OH-}$. So given the same concentration of $\ce{HI}$ and $\ce{HCl}$ in water, they will indeed have the same pH.

In other solvents, you may have a difference between acids, depending on the pKa.

  • $\begingroup$ I don't think there is a lowest pKa in a particular solvent. This study dx.doi.org/10.1063/1.1739994 quantifies the pKas of nitric and perchloric acid in water by NMR. There is also the method of measuring HCl vapor pressure pubs.rsc.org/en/content/articlepdf/1936/tf/tf9363200743 . Maybe there is a lowest pH or at least a lowest p[H+] just due to the fact that there are only so many H3O+ ions that can fit in a given volume. No way p[H3O+] can be less than -log 55. $\endgroup$
    – DavePhD
    Dec 4, 2015 at 13:33

No, not all strong acids are the same strength

There are more than just 6 strong acid.

The acid dissociation constant ($K_a$) quantifies the strength of an acid. pK is negative log base 10 of $K_a$.

Trifluoromethanesulfonic acid pk <-13

Perchloric acid pK = <-9

HI pK = -9.3

HBr pK = -9

HCl pK = -6

sulfuric acid pK = -3

chloric acid pK = -1

nitric acid pK = -1


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