SAJC H2 Chem 2012 Prelim P3 QP

1 ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry Higher 2 9647/03 Paper 3 18 September 2012 2 hours Candidates answer on separate paper. Additional Materials: Writing paper, Data Booklet READ THESE INSTRUCTIONS FIRST: Write your name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 12 printed pages including this page. [Turn Over

2 [Turn Over Answer any 4 questions. 1. Paracetamol and aspirin are well known drugs to treat pain and to reduce fever. (a) The following are structures of paracetamol and aspirin: N H C O CH3 HO O C O CH3 COOH Paracetamol Aspirin pKa = 9.5 pKa = 3.5 Mr = 151.0 Mr = 180.0 (i) Explain the difference in the pKa values of aspirin and paracetamol. (ii) Phenacetin is an analgesic whose use has largely declined due to its carcinogenic properties. Starting from paracetamol, show the steps involved in the synthesis of phenacetin. In your answer, include the reagents and conditions required for each step, as well as the structure of the intermediate involved. N C CH3 O H O Phenacetin CH3CH2 (iii) Salicin is an anti-inflammatory agent that is closely related in chemical make -up to aspirin. Suggest a chemical test that could be used to distinguish salicin from aspirin. Write a balanced equation for the reaction that occurred. Salicin O O CH2OH OH OH OHCH2OH

3 [Turn Over 1. (a) (iv) Aspirin is supplied as a crystalline solid. A standard solution of aspir in may be made by dissolving aspirin in an organic solvent. Explain why water is not used as a suitable solvent. [9] (b) (i) Write an expression for the acid dissociation constant, Ka, of aspirin. (ii) A tablet of aspirin is swallowed into the stomach. Given that the pH of the stomach is 1, calculate the percentage of aspirin molecules that remain undissociated in the stomach. [3] (c) 0.270 g of aspirin and 0.121 g of paracetamol are dissolved in 30 cm 3 of ethanol and titrated against 0.100 mol dm-3 aqueous sodium hydroxide, using 1 to 2 drops of two indicators, methyl red and phenolphthalein. x cm3 of aqueous sodium hydroxide is required to change the colour of the methyl red and a further y cm3 of aqueous sodium hydroxide is needed to change the colour of phenolphthalein. (i) Explain why both indicators are added in small amounts. (ii) Calculate the initial pH of the mixture, assuming the initial pH is from aspirin only.