Titration curves and indicatorsEdexcel International A Level Chemistry: Flashcards

Exam questions on Titration curves and indicators

1. A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with 0.100 mol dm⁻³ sodium hydroxide solution. The Ka of ethanoic acid is 1.74 × 10⁻⁵ mol dm⁻³. Phenolphthalein changes colour over the pH range 8.3–10.0 and methyl orange over the range 3.1–4.4.
   Explain why methyl orange would give an unreliable end point in this titration.2 marks
2. A technician titrates 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid with 0.100 mol dm⁻³ sodium hydroxide in one experiment and with 0.100 mol dm⁻³ ammonia solution in a second experiment. The pH at the equivalence point of the ammonia titration is 5.3. The pH ranges over which some indicators change colour are: methyl red 4.2–6.3; phenolphthalein 8.3–10.0; thymolphthalein 9.3–10.5; alizarin yellow R 10.1–12.0.
   Explain why the pH at the equivalence point of the ammonia titration is below 7.2 marks
3. A student titrates 25.0 cm³ of 0.0500 mol dm⁻³ ethanedioic acid, H₂C₂O₄, a diprotic acid with pKa₁ = 1.25 and pKa₂ = 4.27, with 0.100 mol dm⁻³ sodium hydroxide solution. Methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.
   Describe the key features of the titration curve, including the volumes of sodium hydroxide at the equivalence points.3 marks