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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?
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.
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
