Phenolphthalein changes color over the pH range 8.2–10.0
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Phenolphthalein changes color over the pH range 8.2–10.0. A student uses this indicator to titrate 25.0 mL of 0.100 M acetic acid with 0.100 M NaOH. Which of the following best explains how this choice of indicator affects the calculated concentration of the acid?
(a) Because the indicator’s range overlaps the steep portion of the titration curve, the endpoint and equivalence point coincide, so essentially no systematic error is introduced.
(b) Because the color change occurs before the stoichiometric point, the measured volume of base is smaller than the true equivalence volume, so the calculated acid concentration is slightly too low.
(c) Because the color change occurs after the stoichiometric point, the measured volume of base is slightly larger than the true equivalence volume, so the calculated acid concentration is slightly too high.
(d) Because weak acids react slowly with strong bases, the indicator changes colour exactly at the equivalence point regardless of its pH range, so no error is introduced.