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Définition of pKa

From the Ka we define the pKa which is written pKa = – log Ka

Pka is generally used rather than Ka to determine the strength of an acid.
We notice that the stronger the acid the smaller the pKa. Indeed, if it is strongly dissociated, the concentration [AH] becomes low.

The relation becomes by introducing the logarithms in the relation of Ka :
-log [Ka] = – log [H+] – log[A]/[AH] → pKa = pH – log[A]/[AH] therefore the definition of the pH as a function of the pKa :
pH = pKa + log[A]/[AH]