# How would you compare a strong acid and a weak acid in terms of the acid dissociation constant?

Apr 4, 2017

For a strong acid, by definition, ${K}_{a}$, the acid dissociation constant is LARGE.........

#### Explanation:

A strong acid is an acid for which the following equilibrium lies strongly to the RIGHT as we face the page:

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

And as with any equilibrium, we can quantify the expression by using the relationship:

${K}_{\text{eq}} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{X}^{-}\right]}{\left[H X\right]}$

Now, by definition, for a strong acid, the given equilibrium lies to the right, and thus the NUMERATOR in the expression is LARGE because the product $\left[{H}_{3} {O}^{+}\right] \times \left[{X}^{-}\right]$ is LARGE, and necessarily, the DENOMINATOR is SMALL.

Strong acids in water include the mineral acids ${H}_{2} S {O}_{4}$, $H X \left(X \ne F\right)$, $H C l {O}_{4}$ and dissociation is quantitative.