Final answer:
Option d, HCN and NaCN would form a buffer in an aqueous solution because they consist of a weak acid (HCN) and its conjugate base (the cyanide ion from NaCN), which are necessary components for buffer solutions.
Step-by-step explanation:
The question asks which set of compounds would form a buffer in an aqueous solution. A buffer solution can resist changes in pH upon the addition of an acid or a base and is made from a weak acid and its conjugate base or a weak base and its conjugate acid. Among the given options, d. HCN and NaCN would form a buffer solution because HCN is a weak acid, and NaCN provides the conjugate base, the cyanide ion (CN−).
To create an effective buffer, we need a weak acid or base that can donate or accept protons (H+ ions) and its corresponding salt, which contains the conjugate base or acid. Option a, HF and KF, cannot create a buffer because HF is a weak acid, but KF is not its conjugate acid or base. Option b, NAF and NaOH, does not contain a weak acid-base pair; NaOH is a strong base, which disqualifies it. Option c, NaCl and KCl, includes only salts of strong acids and bases and therefore cannot form a buffer. In conclusion, d. HCN and NaCN would form a buffer when dissolved in an aqueous solution due to the presence of a weak acid and its conjugate base.