CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
H₃D is a triprotic acid with pKa1=2.12, pKa2=2.38, and pKa3=9.60. A 40.0 mL sample of 0.100 M H₃D is titrated at 25 °C with 0.200 M NaOH.
At a certain point in the titration the pH of the mixture is measured to be 2.25.
Answer the following questions regarding this specific mixture.
(a) Calculate the total volume of NaOH that has been added when the pH is 2.25.
(b) Determine the fraction of the acid present in each protonation state (H₃D, H₂D⁻, HD²⁻, and D³⁻) at this pH.
(c) Explain why the first two deprotonation steps cannot be treated as entirely separate in constructing the titration curve for this system.
(d) Predict qualitatively how a plot of pH versus fraction of titrant added would differ from that of a triprotic acid whose first two pKₐ values differ by more than 3 units.