SAJC_H2_CHEM_P2_ANS Prelim

1 [Turn Over Name: Class: ST ANDREW’S JUNIOR COLLEGE JC 2 Preliminary Exam (TUTOR’S COPY) Chemistry Higher 2 9647/02 1 September 2015 Paper 2 2 hours Candidates answer in the spaces provided on the question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. 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. 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 question. For Examiner’s Use: Question Marks 1 2 3 4 5 Total (72 marks) This document consists of 21 printed pages (including this page) and 1 blank page.

2 [Turn Over 1. Planning Calcium hydroxide, Ca(OH) 2, is sufficiently insoluble in water such that it forms a saturated Ca(OH) 2 solution, as shown in the following equation: Ca(OH)2(s) Ca2+(aq) + 2OH-(aq) The following experiments were carried out to determine the solubility product of calcium hydroxide. Different volumes of NaOH and water were added separately to different dry conical flasks according to the following table. Experiment Vol of NaOH / cm3 Vol. of water / cm3 [OH-] in the saturated solution/ mol dm-3 1 5.00 45.00 0.04 2 10.00 40.00 0.03 3 15.00 35.00 0.02 4 20.00 30.00 0.01 5 25.00 25.00 0.001 Excess solid calcium hydroxide was added to each conical flask before it was stoppered and shaken for a few minutes. The solution was filtered and titrated to determine the concentration of hydroxide ions in the saturated solution. (a) On the grid provided, plot a graph using the values from the above table. [1] 0 0.01 0.02 0.03 0.04 0.05 0.06 0 5 10 15 20 25 30 [OH-] in the saturated solution/ mol dm-3 Vol of NaOH / cm3

3 [Turn Over 1(b) Explain why the concentration of hydroxide ions in the saturated solution decreases when more sodium hydroxide is added to the conical flask. [1] [OH-] increases, hence equilibrium to shift to the left . (c) Explain why dry conical flasks needs to be used to prepare the saturated solution. [1] To ensure that the concentration of OH - used is accurate/ dilution will occur/ affect the [OH-] (d) Write the Ksp expression for Ca(OH)2, stating its units. [2] Ksp = [Ca2+(aq)] [OH-(aq)]2 Units: mol3 dm-9 (e) Using the graph plotted, calculate the numerical value of Ksp of Ca(OH)2 [2] Extrapolate the line to the y axis to determine the concentration of [OH-(aq)] when no NaOH(aq) is added. [OH-(aq)] = 0.05 mol dm-3 [Ca2+] = 0.05/2 = 0.025 mol dm-3 Ksp = [Ca2+] [OH-