# How do you write Ka?

Aug 9, 2016

You mean the $\text{acid dissociation constant} , {K}_{a}$?

#### Explanation:

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

Given that $\left[{H}_{2} O\right]$ is effectively a constant, we can write the equilibrium expression in this way:

${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{A}^{-}\right]}{\left[H A\right]}$

And thus for strong acids, where the equilibrium as written lies strongly to the right, ${K}_{a}$ is large. ${K}_{a}$ is small for weaker acids where the equilibrium lies to the left.