A student performs an acid–base titration using a standard solution of NaOH to…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student performs an acid–base titration using a standard solution of 0.100M NaOH to determine the molarity of a monoprotic hydrochloric acid sample. The indicator phenolphthalein is used, producing a faint pink color that persists for 30 s. After completing three trials, the student notices that the pink color appears two or three drops *after* the theoretical equivalence volume predicted from the stoichiometry.
Answer the following questions about the experimental outcome.
(a) Describe the difference between the *equivalence point* and the *end point* of a titration in terms of both stoichiometry and observable phenomena.
(b) Classify the error introduced when the indicator changes color after the equivalence point as *systematic* or *random*. Justify your classification.
(c) Assuming the student always adds approximately 0.10mL of excess NaOH past the equivalence point in each trial, predict whether the calculated molarity of the HCl is too high, too low, or unaffected. Support your prediction with reasoning based on the titration calculation.
(d) Propose two specific procedural modifications the student could implement in future trials to reduce the magnitude of this error and explain how each modification addresses the problem.