EJC Prelim P3_with_Answer

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2018 General Certificate of Education Advanced Level Higher 2 CHEMISTRY Paper 3 Free Response 9729/03 19 September 2018 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on 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 paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a fresh piece of answer paper. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 13 printed pages and 1 blank page.

2 © EJC 9729/03/J2PE/18 Section A Answer all the questions in this section. 1 This question concerns acid-base properties of organic compounds and compounds of some elements in the Periodic Table. (a) The behaviour of a molecular specie as an acid or base is dependent of the surrounding medium and its interaction with another reacting specie. Using the symbol HZ to represent a Brønsted-Lowry acid and the symbol B – to represent a Brønsted-Lowry base, write equations to show the following: (i) CH 3OH acting as a Brønsted-Lowry base. [1] (ii) NH 3 acting as a Brønsted-Lowry acid. [1] (b) In an acid-base reaction, conjugate acid-base pairs can be determined. Copy the table below onto your writing paper and complete it by writing the formula of the conjugate acid-base pairs of the species listed, assuming the acids/bases are monobasic/monoacidic in nature respectively. acid base I H2O II H2 [1] (c) A particular weak monobasic acid HA has a dissociation constant, Ka, of 1.0×10–5 mol dm–3. The un-ionised acid molecules are blue and the anions are yellow. (i) Calculate the pH at which a solution containing HA molecules and A – ions is green, i.e. midway between blue and yellow. [1] (ii) Describe qualitatively how the answer in (c)(i) would be affected by the addition of solid sodium salt of the acid, NaA. [2] A solution mixture is prepared by adding 50 cm 3 of 0.10 mol dm –3 of the acid HA to 50 cm3 of 0.10 mol dm–3 of the sodium salt NaA. (iii) Calculate the final pH of the above solution mixture if 0.04 g of NaOH(s) was added. [3]

3 © EJC 9729/03/J2PE/18 [Turn Over (d) Elements and their compounds of the Periodic Table display varied chemical and physical properties. (i) The peroxide ion has the formula 2 2O − . Group 2 peroxides, on heating, decompose to the oxides and oxygen as exemplified by