A 50.0 mL sample of 0.100 M H₃A (a triprotic weak acid) is titrated with 0.100 M NaOH…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 50.0 mL sample of 0.100 M H₃A (a triprotic weak acid) is titrated with 0.100 M NaOH at 25 °C. The pKₐ values of the acid are 2.10, 6.80, and 11.4.
A pH‒volume curve is obtained. Beginning at pH ≈1.2, the curve shows three distinct steep rises separated by two relatively flat regions. The first steep rise centers at V≈50 mL added base; the second steep rise centers at V≈100 mL; the final steep rise occurs near V≈150 mL and levels off above pH 12.
Answer the following questions about the titration curve.
(a) Identify the volumes of NaOH added that correspond to each equivalence point. Justify your choices using stoichiometric relationships.
(b) Describe why the buffer region between the first and second equivalence points is noticeably shorter than the buffer region preceding the first equivalence point.
(c) At what approximate pH values does the curve flatten at each half-equivalence point? Explain how these pH values relate quantitatively to the acid-dissociation constants given.
(d) The first two pKₐ values differ by less than 5 units. Explain qualitatively how this affects the shape of the titration curve and the experimental difficulty of locating the second equivalence point.