# What is Ka?

Oct 12, 2016

${K}_{a}$ is the acid dissociation constant that quantifies the behaviour of an acid in aqueous solution.

#### Explanation:

For acid dissociation in water we can write the general equation:

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

For strong acids, i.e. $H N {O}_{3} , H X , {H}_{2} S {O}_{4}$, the equilibrium lies strongly to the right, and we can normally treat these solutions as stoichiometric in ${H}_{3} {O}^{+}$.

But for weaker acids, e.g. $H N {O}_{2} , {H}_{3} C - C {O}_{2} H$, at equilibrium, some of the acid will remain undissociated and we use ${K}_{a}$ to quantify the equilibrium:

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

And thus for weak acids, ${K}_{a}$ is small, whereas for stronger acids, such as those listed above, ${K}_{a}$ is large, and the acid may undergo almost complete dissociation.