MI Prelim P3 QP

Class Adm No Candidate Name: This question paper consists of 11 printed pages and 1 blank page. 2018 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/03 Paper 3 Free Response 18th Sept 2018 2 hours Candidates answer on separate paper. Additional materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A – Answer all questions. Section B – Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question Section A Section B Total 1 2 3 4 5 Marks 23 20 17 20 20 80

2 Section A Answer all questions from this section. 1 Oxalic acid is a weak diprotic Br Ønsted acid with p Ka1 and p Ka2 values of 1.25 and 4.14 respectively. 25 cm 3 of 0.50 mol dm –3 oxalic acid was titrated against a solution of 1 mol dm –3 of sodium hydroxide. Only 1 to 2 drops of phenolphthalein indicator was used in this titration. It was found that the first colour change occurred at 19.40 cm3. A sketch (not drawn to scale) of the pH titration curve is shown below. (a) Suggest why the value of pKa2 is larger than pKa1. [2] (b) (i) Calculate the value of pH at point W. [1] (ii) Given that the pH at point X is 1.25, calculate the volume of NaOH added at point X. [2] (c) (i) Define the term buffer. [1] (ii) Write an equation to illustrate how the reaction mixture found at point Y can maintain the pH of a solution when a small amount of OH–(aq) is added. [1] (d) (i) Describe how you would recognise the end point of the titration described. [1] (ii) Explain why only 1 or 2 drops of phenolphthalein indicator was used in the titration. [1] pKa1 ⇌ volume of NaOH added / cm3 pKa2 ⇌ pH Y X W

3 [Turn over The oxalate ion, C 2O42−, is known to form complexes with transition metal ions such as platinum(II). The structure of the oxalate ion is as follows. (e) (i) It is observed that a solution of aqueous platinum( II) ions turns universal indicator orange. Given that platinum( II) is a transition metal ion known to form square planar complexes, explain the observation seen with the aid of a chemic