A student performing an acid–base titration insists that the first faint
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student performing an acid–base titration insists that the first faint, permanent color change observed when phenolphthalein is used must be the exact point of stoichiometric neutralization.
Assume the titration involves a monoprotic strong acid with a standardized strong base and that the laboratory is following proper technique otherwise.
(a) Define the terms "equivalence point" and "endpoint" as they apply to an acid–base titration.
(b) Describe two distinct chemical or instrumental reasons why the endpoint observed with an indicator such as phenolphthalein may not coincide exactly with the equivalence point.
(c) Suppose the endpoint occurs at a burette reading that is 0.15 mL smaller than the true equivalence volume. Explain qualitatively (no calculations required) whether the acid concentration calculated from the student’s data will be too high, too low, or unchanged. Justify your answer in terms of the stoichiometry of the titration.