2019 EJC Prelim H2 Chem P3 (solutions with examiner_s comments)

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2019 General Certificate of Education Advanced Level Higher 2 CHEMISTRY Paper 3 Free Response 9729/03 20 September 2019 2 hours Candidates answer on answer booklet. Additional Materials: Answer Booklet 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 staplers, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a fresh page of the answer booklet. 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/19 Section A Answer all the questions from this section. 1 Phosphoric(V) acid , H3PO4, is widely used in the production of fertilizers and the acidification of foods and beverages such as soft drinks. It behaves as a tribasic acid. (a) Phosphoric(V) acid can be formed from the reaction of phosphorous( V) oxide, P4O10, or phosphorous(V) chloride, PCl5, with water. (i) Write an equation for the reaction that occurs when a small amount of PCl5 is dissolved in water. State the expected observations and the colour of the resultant solution when a few drops of Universal Indicator is added. [2] PCl5(s)+ 4H2O(l)  H3PO4(aq) + 5HCl(aq) White fumes observed, red solution (ii) Phosphorus(V) chloride dissolves in some inert pol ar solvents to form solutions which conduct electricity. This is due to the presence of the two ions, [PCl4]+ and [PCl6]. Draw structure of the [PCl6] ion, indicating the types of bonding within the ion, and state its shape. [2] octahedral Comments:  The equation was generally well -attempted, although some students gave the equation for the reaction with limited water (instead of excess water). This was not accepted as the question stated “small amount of PCl5”.  Many students missed out the expected observations, or assumed that effervescence would be given out. Comments:  The most common error for the structure omitted the use of one dative bond. Some others used 6 dative bonds which would result in several electron-deficient chlorine atoms.  The shape around the central atom was generally well-attempted.  Candidates are advised to learn how to sp ell specific technical terms (including “octahedral”).

3 © EJC 9729/03/J2PE/19 [Turn Over (b) In an experiment, 25.0 cm 3 of 0.080 mol dm –3 phosphoric(V) acid is titrated against 0.100 mol dm–3 NaOH. The initial pH of the solution was 1.69. Assu