Titration curves and indicatorsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student titrates 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid with 0.100 mol dm⁻³ sodium hydroxide solution from a burette, recording the pH with a calibrated meter at 298 K, where $K_w$ = 1.00 × 10⁻¹⁴ mol² dm⁻⁶.

   (a)

   What is the pH of the solution at the equivalence point?

   [1 mark]

   • A1.0
   • B7.0
   • C13.0
   • D8.7

   (b)

   What is the pH of the mixture after 30.0 cm³ of sodium hydroxide solution has been added?

   [1 mark]

   • A2.04
   • B13.00
   • C11.96
   • D12.30

   (c)

   Describe the change in pH on adding alkali close to the equivalence point and explain why it happens.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student titrates 25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid (pKₐ = 4.76) with 0.100 mol dm⁻³ sodium hydroxide solution at 298 K. The pH at the equivalence point is about 8.7. The indicators available are methyl orange (colour change over pH 3.2–4.4), bromothymol blue (pH 6.0–7.6) and phenolphthalein (pH 8.2–10.0).

   (a)

   Which indicator is most suitable for this titration?

   [1 mark]

   • AMethyl orange
   • BBromothymol blue
   • CAny of the three indicators
   • DPhenolphthalein

   (b)

   What is the pH of the solution after 12.5 cm³ of sodium hydroxide solution has been added?

   [1 mark]

   • A4.76
   • B2.88
   • C8.73
   • D7.00

   (c)

   Explain why methyl orange is not suitable for this titration.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student titrates 25.0 cm³ of 0.100 mol dm⁻³ of an unknown weak monobasic acid, HX, with 0.100 mol dm⁻³ sodium hydroxide solution, using a pH meter. After adding 12.5 cm³ of sodium hydroxide solution the pH is 3.80, and the equivalence point is reached after 25.0 cm³.

   (a)

   Determine $K_a$ for the acid HX. Explain your method.

   [3 marks]

   (b)

   Between about 5 cm³ and 20 cm³ of added sodium hydroxide solution the pH changes only slowly. Explain, with an equation, how the mixture in the flask in this region resists a change in pH when a small amount of alkali is added.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Students investigate neutralisation using 1.00 mol dm⁻³ solutions at the same starting temperature. When 25.0 cm³ of hydrochloric acid is mixed with 25.0 cm³ of sodium hydroxide solution, the temperature rises by 6.8 K. When 25.0 cm³ of ethanoic acid is mixed with 25.0 cm³ of sodium hydroxide solution, the temperature rises by 6.6 K. Assume the density of every solution is 1.00 g cm⁻³ and the specific heat capacity is 4.18 J g⁻¹ K⁻¹. Three titrations will also be carried out, each using 25.0 cm³ of 0.100 mol dm⁻³ acid and 0.100 mol dm⁻³ base: hydrochloric acid with sodium hydroxide, hydrochloric acid with ammonia solution, and ethanoic acid with ammonia solution.

   (a)

   Calculate the enthalpy change of neutralisation for each acid with sodium hydroxide, and explain why the two values are different.

   [6 marks]

   (b)

   Evaluate whether methyl orange (pH 3.2–4.4) or phenolphthalein (pH 8.2–10.0) could be used to find the end point in each of the three titrations.

   [6 marks]

   Total for question 4: 12 marks

End of questions