# How does Ka relate to acid strength?

Aug 1, 2016

Strong acids typically possess large ${K}_{a}$ values.

#### Explanation:

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

$H X \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {X}^{-}$

And of course ${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{X}^{-}\right]}{\left[H X\right]}$

For strong acids, i.e. $H X , {H}_{2} S {O}_{4.} H C l {O}_{4}$, 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 $\left[{H}_{3} {O}^{+}\right]$ and $\left[{X}^{-}\right]$ are large and $\left[H X\right]$ is small, then ${K}_{a}$ is correspondingly large. And thus large ${K}_{a}$ values betoken strong acids.