pH curves, titrations and indicatorsAQA A-Level Chemistry: Flashcards

Exam questions on pH curves, titrations and indicators

1. A student titrates 25.0 cm³ portions of sodium hydroxide solution of unknown concentration with 0.0500 mol dm⁻³ hydrochloric acid from a burette. The mean titre is 21.60 cm³.
   Explain why either methyl orange or phenolphthalein is suitable as the indicator for this titration.2 marks
2. Four titrations are carried out. In each, 25.0 cm³ of a 0.100 mol dm⁻³ acid is in the flask and a 0.100 mol dm⁻³ base is added from the burette. Titration 1: hydrochloric acid with sodium hydroxide. Titration 2: ethanoic acid with sodium hydroxide. Titration 3: hydrochloric acid with ammonia solution. Titration 4: ethanoic acid with ammonia solution. The pH is recorded throughout with a pH meter. Data given: methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.
   Explain why no indicator would give a sharp end point in titration 4.2 marks
3. A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid (Ka = 1.74 × 10⁻⁵ mol dm⁻³) with 0.125 mol dm⁻³ sodium hydroxide solution from a burette. Methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.
   Describe the main features of the pH curve for this titration, giving approximate pH values.3 marks