Answer
(a) Buffer;
(b) Not a buffer;
(c) Buffer;
(d) Not a buffer;
Work Step by Step
1. Identify the conjugate acid-base pairs for each solution.
(It has to be a weak acid-base one.)
(a) $CN^-$ and $HCN$;
(b) Doesn't have one.
(c) $NH_3$ and $N{H_4}^+$;
(d) Doesn't have one.
Since the solutions have similar concentrations for both acids and bases, (a) and (c) are buffers.
(b) The common ion here is $Na^+$, which isn't a significant electrolyte.
(c) The common ion here is $OH^-$, which is strong, so the solution can't be a buffer.
