CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A 40.0 mL sample of 0.075 M aqueous ammonia, NH₃, is titrated with 0.150 M HCl at 25 °C. The base-dissociation constant of ammonia is Kb=1.8×10−5. Pertinent indicator data are shown.
Indicator
pKa (25 °C)
Color-change interval (pH)
Bromocresol green
4.9
3.8 – 5.4
Bromothymol blue
7.1
6.0 – 7.6
Phenolphthalein
9.3
8.2 – 10.0
For phenolphthalein, the pKa increases by 0.35 units when the titration temperature is raised from 25 °C to 40 °C.
(a) Calculate the volume of HCl required to reach the equivalence point.
(b) Determine the pH at the equivalence point.
(c) At 25 °C, select the most appropriate indicator from the table and justify your choice quantitatively.
(d) If the titration were instead carried out at 40 °C, evaluate whether the same indicator would remain suitable. Support your answer with calculations or reasoning based on the temperature-dependent pKa change supplied.