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Diss Factsheets

Physical & Chemical properties

Dissociation constant

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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


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.