CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
During a laboratory exercise, two groups titrate 25.00 mL samples of 0.100 M NH₃(aq) with 0.100 M HCl(aq). Group 1 uses phenolphthalein (transition range pH 8.2–10.0) as the indicator, whereas Group 2 uses methyl orange (transition range pH 3.1–4.4). The pKb of NH₃ is 4.75.
(a) Calculate the pH at the equivalence point for the titration of NH₃ with HCl.
(b) Identify which group’s indicator will produce an end point closest to the equivalence point, and justify your choice with reference to numerical values.
(c) For the group whose indicator changes color appreciably after the equivalence point is passed, predict whether the molarity of NH₃ calculated from the titration data will be reported as too high, too low, or correct. Explain your reasoning.
(d) Describe one procedural modification (other than changing the indicator) that either group could implement to minimize the titration error associated with the mismatch between end point and equivalence point.