CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
An aqueous solution of ammonia, NH₃(aq), is titrated at 25 °C with 0.200 M HCl. A laboratory computer records the following key points of the titration curve.
• Initial 25.00 mL of NH₃ solution: pH = 11.25
• 12.45 mL of HCl added: pH = 9.25
• 24.90 mL of HCl added: pH = 5.13 (steep inflection center)
• 30.00 mL of HCl added: pH = 2.85
(a) Determine the molarity of the initial NH₃ solution.
(b) Calculate Kb and pKb for NH₃.
(c) Determine Ka for NH₄⁺.
(d) Calculate the pH at stoichiometric equivalence.
(e) Discuss the relative impact of ±0.05 pH uncertainty on Kb and the equivalence-point pH.