Question

Which is a stronger base: cyanide ion or fluoride ion? Give pK_b values (without looking them up) to support your choice.

Short answer

The stronger base is the cyanide ion.

pK_b(CN^-) = 4.69

pK_b(F^-) = 10.80

Step-by-step solution

The Acids and Bases.

Acid-base reactions are very quick. They react as soon as they are added to each other in a container. Here, acids release proton that is accepted by bases.

The pKb values and basicity.

The lower the value, and the higher the value, the stronger the basicity.

The stronger base.

A strong acid contains a weak conjugate base and weak acid carries a strong conjugate base.

The same thing goes for bases.

Out of the given ions; the cyanide ion is a stronger base because its conjugate acid is weaker than the latter.

Acids;

pK_a(HCN) = 9.31

pK_a(HF) = 3.20

HF is the stronger acid.

Bases:

pK_b(CN^-) = 4.69

pK_b(F^-) = 10.80

CN^-ion is the stronger base.