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EC number: 231-595-7 | CAS number: 7647-01-0
Dissociation constant
Administrative data
Link to relevant study record(s)
Description of key information
7647-01-0, Hydrogen Chloride,
The endpoint is waived:The study is scientifically impossible, HCl is a very strong acid that dissociates completely in water and therefore the pKa is infinitely.
Key value for chemical safety assessment
Additional information
http://www.chembuddy.com
Values of dissociation constants pKa and pKb for acids and bases.mht
Accuracy of the values for the PKa of HCl and other strong acids, is questionable. Depending on the source pKafor HCl is given as -3, -4 or even -7. These values are usually not measured but calculated from thermodynamical data and should not be treated too seriously. Besides, difference between pKa=-1 and pKa=-10 influences calculation results for the solutions with high ionic strength, which are dubious in any case.
In reality
it does not matter in aqueous solution, since everything will be
converted into hydronium and chloride ion-, that is why we write the
reaction in one direction only.
HCl (aq) ==> H+ + Cl-
The acid is 100% dissociated, so every single last molecule of HCl will
be converted into H+ and Cl-. For strong acids in the gas phase it does
make a difference, but if you are dealing with aqueous solutions,
talking about Ka values for strong acids is irrelevant.
That is why for any strong acid, the pH of the solution is -log([strong
acid]), so solutions of 0.1 M HCl, HNO3 and HClO4, all will give you the
same pH, 1.0.
Any strong acid when dissolved in water is converted into H+, since H+
is the strongest acid that can exist in water. That is called the
leveling effect.
In essence, using this approach, you can say that the Ka of any strong
acid approaches infinity, any number divided by zero is in essence
infinity.
The Ka of any strong acid is irrelevant in the calculations that you
have to perform using that strong acid.
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