A 25.0mL sample of a solution of an unknown compound is titrated with a 0.115M NaOH solution. The following titration curve is obtained. The unknown compound is _____.
options:A) a weak acidB) a strong acidC) a strong baseD) neither an acid nor a baseE) a weak base
The correct answer and explanation is:
Correct Answer: A) a weak acid
To determine the identity of the unknown compound in a titration, the titration curve provides crucial information about the pH changes as a titrant is added. In this case, the titrant is a 0.115 M NaOH solution, which is a strong base. The NaOH reacts with the unknown compound, and the resulting pH changes are plotted to generate a titration curve.
If the unknown compound were a strong acid, the initial pH would be very low (close to 1), and the pH would rise sharply and almost vertically at the equivalence point. The equivalence point for a strong acid–strong base titration occurs at pH 7.
If the unknown compound were a weak acid, the titration curve would begin at a moderately low pH (typically between 3 and 5), and the curve would rise more gradually. The key feature would be a buffering region in the beginning, followed by a more gradual increase in pH. The equivalence point for a weak acid titrated with a strong base typically occurs above pH 7, often between 8 and 9. The presence of this shift indicates the acid was not fully dissociated to start with, which is a characteristic of weak acids.
If the unknown were a strong base, the titration would begin at a very high pH and decrease as a strong acid is added, which does not match this setup. A weak base titrated with a strong acid would have a starting pH in the range of 9 to 11 and an equivalence point below pH 7.
If the compound were neither an acid nor a base, the pH would not change significantly during titration.
Given that NaOH is used and a titration curve is observed with a gradual rise and an equivalence point above pH 7, the unknown compound is best identified as a weak acid.