Answer:
[tex]\frac{[F^{-}]}{[HF]}[/tex] is larger
Explanation:
[tex]pK_{a}=-logK_{a}[/tex] , where [tex]K_{a}[/tex] is the acid dissociation constant.
For a monoprotic acid e.g. HA, [tex]K_{a}=\frac{[H^{+}][A^{-}]}{[HA]}[/tex] and [tex]\frac{[A^{-}]}{[HA]}=\frac{K_{a}}{[H^{+}]}[/tex]
So, clearly, higher the [tex]K_{a}[/tex] value , lower will the the [tex]pK_{a}[/tex]
In this mixture, at equilibrium, [tex][H^{+}][/tex] will be constant.
[tex]K_{a}[/tex] of HF is grater than [tex]K_{a}[/tex] of HCN
Hence, [tex](\frac{F^{-}}{[HF]}=\frac{K_{a}(HF)}{[H^{+}]})>(\frac{CN^{-}}{[HCN]}=\frac{K_{a}(HCN)}{[H^{+}]})[/tex]
So, [tex]\frac{[F^{-}]}{[HF]}[/tex] is larger
