Titration curves and indicatorsEdexcel International A Level Chemistry: Subtopic test

1. 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.

   (a)

   What is the approximate pH at the equivalence point of this titration?

   [1 mark]

   • Aabout 4.8
   • Bexactly 7.0
   • Cabout 13.0
   • Dabout 8.7

   (b)

   Which indicator is suitable for this titration?

   [1 mark]

   • Amethyl orange only
   • Bphenolphthalein only
   • Cboth indicators
   • Dneither indicator

   (c)

   Explain why methyl orange would give an unreliable end point in this titration.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which indicator is suitable for the titration of hydrochloric acid with ammonia solution?

   [1 mark]

   • Amethyl red
   • Bphenolphthalein
   • Cthymolphthalein
   • Dalizarin yellow R

   (b)

   In the sodium hydroxide titration, what is the pH after 15.0 cm³ of the alkali has been added?

   [1 mark]

   • A1.00
   • B1.40
   • C1.60
   • D7.00

   (c)

   Explain why the pH at the equivalence point of the ammonia titration is below 7.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe the key features of the titration curve, including the volumes of sodium hydroxide at the equivalence points.

   [3 marks]

   (b)

   Calculate the concentration of ethanedioate ions at the second equivalence point. Hence explain which of the two indicators is suitable for the second equivalence point and why the other is not.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student uses a pH meter to follow the titration of 25.0 cm³ portions of two acids, each with 0.100 mol dm⁻³ sodium hydroxide. One acid is 0.100 mol dm⁻³ hydrochloric acid. The other is an ethanoic acid solution of unknown concentration, which needs 22.5 cm³ of the sodium hydroxide to reach the end point. The pH of the ethanoic acid titration was 4.76 after 11.25 cm³ of alkali had been added.

   (a)

   Compare the shapes of the titration curves of hydrochloric acid and ethanoic acid with sodium hydroxide, and discuss which indicators could be used for each.

   [6 marks]

   (b)

   Use the data to calculate the concentration and the Ka of the ethanoic acid, and hence the pH of the ethanoic acid solution before any alkali is added.

   [6 marks]

   Total for question 4: 12 marks

End of questions