# How do you write an expression for the acid ionization constant (K_a) for HF?

$H F \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {F}^{-}$
...and this reaction is not so complete as the lower hydrogen halides in that $H - F$ is certainly quite strong, and ALSO, the ${F}^{-}$ is disfavoured entropically in aqueous solution.
And so we write ${K}_{a} = \frac{\left[{H}_{3} {O}^{+}\right] \left[{F}^{-}\right]}{\left[H F \left(a q\right)\right]} = 7.2 \times {10}^{-} 4$...