RI Prelim P3

Page 1 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 RAFFLES INSTITUTION (JUNIOR COLLEGE) PRELIMINARY EXAMINATION 2009 HIGHER 2 CHEMISTRY 9746/03 Paper 3 Free Response 16 September 2009 2 hours Candidates answer on separate paper. Additional Materials: Writing Papers Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, civics tutorial group and index nu mber in the spaces provided on the cover page on page 9 and the writing papers. Write in dark blue or black pen on both sides of pa per. 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. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good English and c lear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together, with the cover page on top. ________________________________________________________________________ This question booklet consists of 8 printed pages and one cover page.

Page 2 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 Answer any four questions. Begin each question on a fresh sheet of paper. 1 Lactic acid (2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. (a) 25.0 cm 3 of an aqueous solution of lactic acid is titrated against 0.120 mol dm − 3 of aqueous sodium hydroxide and the following graph is obtained. 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 0 10 20 30 40 Volume of aq NaOH added/ cm pH 3

Page 3 © RAFFLES INSTITUTION (JUNIOR COLLEGE) 2009 9746/P03/09 (i) Calculate the concentration of the lactic acid used. (ii) Determine the acid dissociation constant of lactic acid. (iii) With the aid of an equation, explain why the pH at equivalence point is above 7. [4] (b) The table below lists some properties of three samp les of lactic acid prepared from different natural sources. Sample Source Effect on plane− polarised light Melting point/ oC A Meat extract Rotate to the right 26 B Fermentation of sucrose Rotate to the left 26 C Sour milk No effect 18 (i) With the help of suitable structural formulae, exp lain why samples A and B behave differently towards plane− polarised light. (ii) Explain why sample C has no effect on plane−polarised light and suggest why it has a different melt