Question

of the following anions, which is the strongest base?

A) I-

B) CN-

C) NO3-

D) Br-

Answer 1

The strongest base will be the conjugate one of the weakenst acid. The acids of these bases are, respectively, HI, HCN, HNO3 and HBr. Now, due to size mismatching between H and Br or, I (say, X), H-X bond is weak. So, HI and Hbr are strong acids. HNO3 is also a strong acid because the NO3- ion is more stabilized by resonance. Thus, the only option left is HCN. It is weak acid, because-

• negative charge on carbon atom in CN- is not so much stable, like other cases.
• size matching between H and C is highest, providing a less facile H-C bond dissociation.

So, the correct option is B.