Year 2 - Acids and Bases Q1. The value of the acid dissociation constant, K., for ethanoic acid is 1.74 x 10-5 mol dm-3 at 298 K. (a) (i) Write an expression for K, for ethanoic acid. (ii) Calculate the pH at 298 K of a 0.220 mol dm-3 solution of ethanoic acid. (5) (b) A sample of the 0.220 mol dm-3 solution of ethanoic acid was titrated against sodium hydroxide solution. (i) Calculate the volume of a 0.150 mol dm-3 solution of sodium hydroxide required to neutralise 25.0 cm3 of the ethanoic acid solution. (ii) From the list below, select the best indicator for this titration and explain your choice. Name of indicator pH range bromophenol blue 3.0 - 4.6 methyl red 4.2 - 6.3 bromothymol blue 6.0 - 7.6 thymol blue 8.0 - 9.6 Page 1
Year 2 - Acids and Bases Indicator Explanation (c) A buffer solution is formed when 2.00 g of sodium hydroxide are added to 1.00 dm3 of a 0.220 mol dm-3 solution of ethanoic acid. Calculate the pH at 298 K of this buffer solution. (5) (6) (Total 16 marks) Q2. A 0.210 mol dm-3 solution of potassium hydroxide was added from a burette to 25.0 cm3 of a 0.160 mol dm-3 solution of ethanoic acid in a conical flask. Given that the value of the acid dissociation constant, Ka, for ethanoic acid is 1.74 x 10- 5 mol dm-3, calculate the pH at 25 ? of the solution in the conical flask at the following three points: before any potassium hydroxide had been added; after 8.0 cm3 of potassium hydroxide solution had been added; after 40.0 cm3 of potassium hydroxide solution had been added. (Total 16 marks) Page 2
Year 2 - Acids and Bases Q3. (a) Methylamine is a weak Brønsted-Lowry base and can be used in aqueous solution with one other substance to prepare a basic buffer. (i) Explain the term Brønsted-Lowry base and write an equation for the reaction of methylamine with water to produce an alkaline solution. Brønsted-Lowry base Equation (ii) Suggest a substance that could be added to aqueous methylamine to produce a basic buffer. (iii) Explain how the buffer solution in part (a)(ii) is able to resist a change in pH when a small amount of sodium hydroxide is added. (5) (b) Explain why methylamine is a stronger base than ammonia. (2) (c) A cation is formed when methylamine reacts with a large excess of bromoethane. Name the mechanism involved in the reaction and draw the structure of the cation formed. Name of mechanism Page 3