Question

How does Ka relate to acid strength?

Answer 1

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.