How does Ka relate to acid strength?

1 Answer
anor277
Aug 1, 2016

Strong acids typically possess large #K_a# values.

Explanation:

The generalized protonolysis reaction for an acid in water is as follows:

#HX(aq) + H_2O(l) rightleftharpoons H_3O^(+) + X^-#

And of course #K_a=([H_3O^+][X^-])/([HX])#

For strong acids, i.e. #HX, H_2SO_4. HClO_4#, the equilibrium lies almost quantitatively to the RIGHT. Given this we can look back to the the #K_a# expression and realize that if equilibrium quantities of #[H_3O^+]# and #[X^-]# are large and #[HX]# is small, then #K_a# is correspondingly large. And thus large #K_a# values betoken strong acids.

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