MI_H2_CHEM_P2_Answer Prelim

Class Adm No Candidate Name: This question paper consists of 11 printed pages and 1 blank page. 2015 Preliminary Examination II Pre-university 3 H2 CHEMISTRY 9647/02 Paper 2 Structured Questions 16th Sept 2015 2 hours Candidates answer on the Question paper. Additional materials: 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. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 1 2 3 4 5 Total Marks 12 10 24 15 11 72

2 1 Planning (P) Ascorbic acid is a naturally occurring organic compound commonly found in citrus fruits and Vitamin C supplements. It is sometimes called an ‘anti-oxidant’ or a reducing agent. Ascorbic acid is water soluble. Direct iodometric titration can be used to determine the amount of ascorbic acid in samples of juices and Vitamin C supplements. Iodine rapidly oxidises ascorbic acid, C6H8O6, to produce dehydroascorbic acid, C6H6O6. C6H8O6 + I2 → C6H6O6 + 2I– + 2H+ In the direct iodometric titration, the ascorbic acid is titrated against an iodine solution of known concentration using starch as an indicator. Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch indicator. A student was given a Vitamin C sample which the ma nufacturer claims to contain 0.25 g of ascorbic acid per tablet. She was tasked to plan an experiment to determine if the manufacturer’s claim was true. The following reagents were provided: FA 1 0.00500 mol dm-3 I2 solution FA 2 starch indicator (a) State the colour at the end-point of the titration. ………………………………………………………………………………………………………… [1] Dark blue/black (b) The student was provided with a 250 cm3 volumetric flask to prepare an ascorbic acid solution using one Vitamin C tablet. The solution was labelled FA 3. Assuming that the manufacturer’s claim is true, calculate the concentration of ascorbic acid in FA 3. [1] Amount of ascorbic acid = 0.25 176.0 = 0.001420 mol [FA3] = 0.001420 0.250 = 0.00568 mol dm-3 (c) Given that the average titre volume should be in the range of 10.0 to 30.0 cm 3, determine the minimum volume of FA 3 that should be used in the experiment.

3 [Turn over Hence, suggest a suitable pipette volume for titration. [3] Using minimum titre volume, amount of I2 in 10.0 cm3