Which one of the following pairs of compounds could NOT be used to form a pH buffer solution in water?

A buffer solution is a solution containing a weak acid and its conjugate base, or a weak base and its conjugated acid..

(a) could be used as a pH buffer solution.
The solution contains a weak base NH₃ and its conjugate acid NH₄⁺ (in NH₄Cl).

(b) could be used as a pH buffer solution.
HCl reacts with excess NaCH₃COO to form CH₃COOH. The resulting solution contains a weak acid CH₃COOH its conjugate base CH₃COO⁻ (in NaCH₃COO).

(c) could be used as a pH buffer solution.
The solution contains a weak acid CH₃COOH its conjugate base CH₃COO⁻ (in NaCH₃COO).

(d) could be used as a pH buffer solution.
KOH reacts with excess HClO to form KClO. The resulting solution contains a weak acid HClO and its conjugate base ClO⁻ (in KClO).

(e) could NOT be used as a pH buffer solution.
CN⁻ is the conjugate base of the weak acid HCN. The solution cannot be used as pH buffer solution due to the absence of HCN.

The answer: (e) NaOH and NaCN