Question

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

Answer 1

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

Explanation:

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.