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 concentration of the original NH₃ solution.
(b) Calculate Kb for NH₃ and pKb using only information from the titration data.
(c) Use your result from part (b) to determine Ka for NH₄⁺.
(d) Calculate the pH of the solution exactly at the stoichiometric equivalence point.
(e) The pH probe used in the experiment has reported error bars of ±0.05 pH units across the entire range. Discuss how this uncertainty propagates through the calculations of Kb and the equivalence-point pH. In your discussion, identify which of the two calculated quantities is more sensitive to the pH measurement uncertainty and explain why.
(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.