1. 1
   Propanoic acid, CH₃CH₂COOH, and its sodium salt, sodium propanoate, are used as food preservatives. At 298 K the pKa of propanoic acid is 4.87 and Kw = 1.00 × 10⁻¹⁴.

   (a)

   What is the formula of the conjugate base of propanoic acid?

   [1 mark]

   • ACH₃CH₂COOH₂⁺
   • BCH₃CH₂COO⁻
   • COH⁻
   • DH₃O⁺

   (b)

   What is the value of Kb for the propanoate ion at 298 K?

   [1 mark]

   • A7.4 × 10⁻¹⁰
   • B1.3 × 10⁻⁵
   • C9.13
   • D1.3 × 10⁻¹⁹

   (c)

   Construct an equation for the hydrolysis of the propanoate ion and predict, with a reason, whether an aqueous solution of sodium propanoate is acidic, neutral or basic.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student titrates 25.0 cm³ of 0.100 mol dm⁻³ benzoic acid, C₆H₅COOH (a weak monoprotic acid, pKa = 4.20), with 0.100 mol dm⁻³ potassium hydroxide solution at 298 K. The pH at the equivalence point is 8.45. Three indicators are available: methyl orange (pKa 3.7, red in acid and yellow in alkali), bromothymol blue (pKa 7.1) and phenolphthalein (pKa 9.3, colourless in acid and pink in alkali).

   (a)

   Which indicator is most appropriate for this titration?

   [1 mark]

   • AMethyl orange
   • BBromothymol blue
   • CAny of the three, because every titration has a large pH change at equivalence
   • DPhenolphthalein

   (b)

   What is the pH when 12.5 cm³ of potassium hydroxide has been added?

   [1 mark]

   • A2.60
   • B7.00
   • C4.20
   • D8.45

   (c)

   Phenolphthalein is a weak acid, HIn. Explain, using an equilibrium equation, why phenolphthalein changes colour close to pH 9.3.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A glass-cleaning solution contains 0.150 mol dm⁻³ aqueous ammonia, NH₃. At 298 K the pKb of ammonia is 4.75 and Kw = 1.00 × 10⁻¹⁴. A technician adds 0.0500 mol of solid ammonium chloride to 1.00 dm³ of this solution without changing its volume.

   (a)

   Calculate the pH of the original 0.150 mol dm⁻³ ammonia solution, before the ammonium chloride is added.

   [3 marks]

   (b)

   Determine the pH of the solution after the ammonium chloride has been added, and explain how this solution resists a change in pH when a small amount of hydrochloric acid is added.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In each of four titrations, the titrant (0.100 mol dm⁻³) was added from a burette to 20.0 cm³ of another 0.100 mol dm⁻³ solution at 298 K, and the pH was recorded with a meter. The acids and bases used were chosen from hydrochloric acid, ethanoic acid (pKa 4.76), sodium hydroxide and aqueous ammonia (pKb 4.75); all are monoprotic.
   Titration 1: initial pH 11.12; pH 9.25 after 10.0 cm³; the pH fell from 7.25 to 3.30 between 19.8 cm³ and 20.2 cm³; pH at equivalence 5.28.
   Titration 2: initial pH 2.88; pH 4.76 after 10.0 cm³; the pH rose from 6.76 to 10.70 between 19.8 cm³ and 20.2 cm³; pH at equivalence 8.73.
   Titration 3: initial pH 1.00; pH 1.48 after 10.0 cm³; the pH rose from 3.30 to 10.70 between 19.8 cm³ and 20.2 cm³; pH at equivalence 7.00.
   Titration 4: initial pH 2.88; pH 4.76 after 10.0 cm³; the pH rose only from 6.76 to 7.25 between 19.8 cm³ and 20.2 cm³; pH at equivalence about 7.0.
   Indicator colour-change ranges: methyl red pH 4.4–6.2; bromothymol blue pH 6.0–7.6; phenolphthalein pH 8.2–10.0.

   (a)

   Deduce the type of acid and base (strong or weak) used in each of titrations 1 to 4, justifying each deduction with the data.

   [6 marks]

   (b)

   Evaluate which of the three indicators is suitable for each of titrations 1, 2 and 3, and explain why none of them is suitable for titration 4.

   [6 marks]

   Total for question 4: 12 marks

End of questions