Question

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

Answer 1

Well we write the equation first off....

Explanation:

`HF(aq) + H_2O(l) rightleftharpoonsH_3O^+ +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=([H_3O^+][F^(-)])/([HF(aq)])=7.2xx10^-4`...