CJC_H2_CHEM_P2_ANSWER Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS 2T CHEMISTRY 9647/02 Paper 2 Structured Questions TUESDAY 13 SEPTEMBER 2011 2 Hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] 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 all questions. A Data Booklet is provided You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part of the question. At the end of the examination, fasten all work securely together. ANSWERS This document consists of 17 printed pages and 1 blank page. [Turn over]

2 9647/02/CJC JC2 Prelim Exam 2011 1 Planning (P) A mixture of sodium carbonate, Na2CO3 and sodium hydrogencarbonate, NaHCO3 is heated strongly with a bunsen burner. Only sodium hydrogencarbonate, NaHCO 3 decomposes on heating. 2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(g) You are to design an experiment to determine the percentage by mass of sodium hydrogencarbonate, NaHCO3 in a 5.00 g sample of a mixture of sodium carbonate, Na 2CO3 and sodium hydrogencarbonate, NaHCO3, by heating alone. The only apparatus available consists of a boiling-tube and holder, a heat-proof mat, a chemical balance and a bunsen burner. (a) Outline step by step, the practical sequence for the method you would use to  make appropriate weighings,  decompose NaHCO 3 in the sample by heating,  ensure that decomposition was complete. Weigh accurately the empty dry boiling-tube, using a chemical/electronic balance. Add 3.00 g of the mixture of the sample and weigh the boiling-tube and contents (or any reasonable specified amount). Heat the sample in the boiling-tube strongly, for about 5 minutes, using a test-tube holder and bunsen burner. Allow the boiling-tube and the contents to cool on a heat-proof mat. After cooling, reweigh the boiling-tube and its contents. Repeat the heating, cooling and reweighing until there is no further loss in mass. This is to ensure that all the NaHCO3 has decomposed completely. [4] (b) Show how you would tabulate your results. Include in your table and other masses you would calculate from the experimental results to enable you to determine the percentage by mass of NaHCO3 in the sample of the mixture. Insert in your table, the letters A, B, C etc. to represent each measurement of mass. Mass of boiling-tube and sample/g B Mass of empty boiling-tube/g A Mass of sample/g B - A Mass of boiling-tube and contents/g after 1st heating/