pH curves, titrations and indicatorsAQA A-Level Chemistry: Subtopic test

1. 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.

   (a)

   How many moles of hydrochloric acid are in the mean titre?

   [1 mark]

   • A1.08 × 10⁻⁶ mol
   • B2.16 × 10⁻³ mol
   • C1.08 × 10⁻³ mol
   • D1.25 × 10⁻³ mol

   (b)

   What is the concentration of the sodium hydroxide solution?

   [1 mark]

   • A0.0216 mol dm⁻³
   • B0.0500 mol dm⁻³
   • C0.0864 mol dm⁻³
   • D0.0432 mol dm⁻³

   (c)

   Explain why either methyl orange or phenolphthalein is suitable as the indicator for this titration.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which titration has an equivalence point above pH 7?

   [1 mark]

   • ATitration 1
   • BTitration 2
   • CTitration 3
   • DTitration 4

   (b)

   Which indicator is suitable for titration 3?

   [1 mark]

   • AMethyl orange only
   • BPhenolphthalein only
   • CBoth methyl orange and phenolphthalein
   • DNeither, because there is no vertical section

   (c)

   Explain why no indicator would give a sharp end point in titration 4.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe the main features of the pH curve for this titration, giving approximate pH values.

   [3 marks]

   (b)

   Calculate the volume of sodium hydroxide solution needed to reach the end point. Select the more suitable indicator, and explain why the other indicator is not suitable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician investigates a household ammonia cleaner. In a practical, 0.100 mol dm⁻³ ammonia solution is added from a burette to 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid and the pH is measured after each addition. The technician then analyses the cleaner: 10.0 cm³ of it is diluted to 250 cm³ in a volumetric flask, and 25.0 cm³ portions of the diluted solution are titrated with 0.100 mol dm⁻³ hydrochloric acid. The mean titre is 23.40 cm³. Methyl orange changes colour over the pH range 3.1–4.4 and phenolphthalein over the range 8.3–10.0.

   (a)

   Describe and explain the shape of the pH curve for adding ammonia solution to the hydrochloric acid, and explain which indicator would be suitable.

   [6 marks]

   (b)

   Calculate the concentration of ammonia in the cleaner in mol dm⁻³ and in g dm⁻³ (Mr of NH₃ = 17.0).

   [6 marks]

   Total for question 4: 12 marks

End of questions