Final answer:
The pH at the equivalence point when titrating HCN with NaOH will be alkaline due to the formation of a basic salt and the excess of OH- ions from the strong base NaOH.
Step-by-step explanation:
When titrating HCN with NaOH, the pH at the equivalence point will be alkaline (c). This is because HCN is a weak acid and NaOH is a strong base. During the titration, the weak acid is neutralized by the strong base, forming water and the sodium salt of the acid (sodium cyanide, NaCN). At the equivalence point, the sodium cyanide undergoes hydrolysis since its conjugate acid (HCN) is weak, resulting in a solution that is basic due to the presence of excess OH⁻ ions. This is supported by titrations of weak acids with strong bases, such as the titration curve for acetic acid and NaOH, where the equivalence point pH is greater than 7. Therefore, after the titration reaches equivalence, the resulting solution will have a pH greater than 7, indicating an alkaline environment.