Answer
An anion will act as a weak base if it is the conjugate base of a weak acid.
$$A^-(aq) + H_2O(l) \leftrightharpoons HA(aq) + OH^-(aq)$$
Work Step by Step
- To determine if an anion will act as a weak base, we have to analyze its conjugate acid.
If the conjugate acid is strong, the anion is not able to act as a base, since the ionization of a strong acid does not have equilibrium and a reverse reaction; for example:
$$HCl(aq) + H_2O(l) \longrightarrow Cl^-(aq) + H_3O^+(aq)$$
Since there is only one arrow, going left to right, it means that the reverse reaction does not occur, meaning that $Cl^-$, the anion of a strong acid, cannot act as a base and accept the proton of $H_3O^+$.
On the other side, a weak acid reacting does have a reverse reaction, meaning that the conjugate base can act as a base.
$$HA(aq) + H_2O(l) \leftrightharpoons A^-(aq) + H_3O^+(aq)$$
- The anion will act as a base, and the most generic reactant is water, so we just have to write an equation where the anion accepts one proton from the water molecule, producing its conjugate acid and the hydroxide ion.
