Answer
a. Does not act as a weak base.
b. Acts as a weak base:
$$ClO^-(aq) + H_2O(l) \leftrightharpoons HClO(aq) + OH^-
(aq)$$
c. Acts as a weak base:
$$CN^-(aq) + H_2O(l) \leftrightharpoons HCN(aq) + OH^-
(aq)$$
d. Does not act as a weak base.
Work Step by Step
The anion will act as a weak base in solution if it is a conjugate base of a weak acid. If it is the conjugate base of a strong acid, it will not ionize.
a. $Br^- \longrightarrow HBr$. Since HBr is a strong acid, $Br^-$ does not ionize.
b. $ClO^- \longrightarrow HClO$. Since HClO is a weak acid, $ClO^-$ acts as a weak base.
c. $CN^- \longrightarrow HCN$. Since HCN is a weak acid, $CN^-$ acts as a weak base.
d. $Cl^- \longrightarrow HCl$. Since HCl is a strong acid, $Cl^-$ does not ionize.
To rewrite the equation, just write the reaction where the anion receives a proton $H^+$ from a water molecule. The reactants are the anions and a water molecule, and the products are the conjugate acid and a hydroxide ion.
