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Which indicator should be used in a titration to find the concentration of a solution of methylamine using 0.010 mol dm⁻³ hydrochloric acid? A - AQA - A-Level Chemistry - Question 32 - 2018 - Paper 3
Question 32
Which indicator should be used in a titration to find the concentration of a solution of methylamine using 0.010 mol dm⁻³ hydrochloric acid? A. Thymol blue (pH ran... show full transcript
Worked Solution & Example Answer:Which indicator should be used in a titration to find the concentration of a solution of methylamine using 0.010 mol dm⁻³ hydrochloric acid? A - AQA - A-Level Chemistry - Question 32 - 2018 - Paper 3
Step 1
Identify the appropriate indicator for the titration
Answer
In this case, we are titrating a solution of methylamine, which is a weak base, with hydrochloric acid, a strong acid. The equivalence point of this titration will occur at a pH lower than 7, generally around pH 5-6 due to the stronger acid being used against the weaker base.
Considering the indicators provided:
- Thymol blue has a pH range of 1.2–2.8, which is too acidic for this titration.
- Bromophenol blue has a pH range of 3.0–4.6, which is closer but still on the lower end.
- Phenol red has a pH range of 6.8–8.4, which is too high for the expected equivalence point.
- Phenolphthalein has a pH range of 8.3–10.0, which is also inappropriate as it indicates the endpoint of a strong base versus a weak acid titration.
Thus, the suitable indicator for the titration of methylamine with hydrochloric acid is Bromophenol blue.