How does Ka relate to acid strength?

1 Answer
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.