A student performs a titration to characterize an unknown monoprotic weak acid, HA
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student performs a titration to characterize an unknown monoprotic weak acid, HA. A 25.00 mL aliquot of the acid solution is delivered into an Erlenmeyer flask and titrated with 0.100 M NaOH from a buret. The indicator phenolphthalein (colorless in acidic solution, pink in basic solution) is used.
During the experiment the solution in the flask changes from colorless to the faintest persistent pink at a volume of 27.36 mL of NaOH added.
(a) Distinguish between the terms "equivalence point" and "end point" for this titration. Include a discussion of how each is detected or defined.
(b) Phenolphthalein changes color over the pH range 8.2–10.0. Explain why this indicator is nevertheless suitable for the titration described.
(c) If the student mistakenly stopped the titration when only 25.80 mL of NaOH had been added, qualitatively describe how the calculated molarity of the acid would compare to the correct value. Justify your answer.