Why does a strong acid have a weak conjugate base, whereas a weak acid has a relatively strong conjugate base?

1 Answer
Jan 1, 2016

Because a strong acid necessarily implies a weak conjugate base, and a strong base implies a weak conjugate acid.


For strong acids, the following equilibrium lies strongly to the right:

#HA + H_2O rightleftharpoons H_3O^+ + A^-#

This implies that the conjugate BASE of #HA#, #A^-# does not compete strongly for the proton. For weaker acids, say hydrogen fluoride, the #H-F# is strong so that #F^-# is moderately basic. A solution of sodium fluoride in water would be slightly basic because the basic fluroide would abstract a proton from water to give back the parent acid:

#F^(-) + H_2O rightleftharpoons HF + HO^-#

A little time and effort taken to understand these definitions and equilibria will be very useful (and I mean you should consult your text). Good luck.