Copy of 2024 SCGS Chemistry Prelim P3

1 CANDIDATE NAME CLASS 4 REGISTER NUMBER CHEMISTRY Paper 3 Practical Friday 6092/03 02 Aug 2024 1 hour 50 min Candidates answer on the Question Paper READ THESE INSTRUCTIONS FIRST Write your name, class and register number on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction tape. Answer all questions in the spaces provided in the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on page 9. 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. SINGAPORE CHINESE GIRLS’ SCHOOL PRELIMINARY EXAMINATION SECONDARY FOUR Shift Laboratory For examiner’s Use 1 / 15 2 / 16 3 / 9 Total / 40 This document consists of 9 printed pages and 1 blank page.

2 1 The aim of this experiment is to determine the relative formula mass of barium peroxide, BaO2. Solution R, which contains hydrogen peroxide, was prepared by adding dilute sulfuric acid to 8.50 g of barium peroxide, filtering off the insoluble barium sulfate and diluting the resulting solution to 1.00 dm3. The equation for the reaction is shown below: BaO2(s) + H2SO4(aq)  BaSO4(s) + H2O2(aq) ----- equation 1 The concentration of hydrogen peroxide in R can be determined by adding acidified aqueous potassium iodide and titrating the liberated iodine with aqueous sodium thiosulfate. The equations for the reactions are shown below: H2O2 + 2KI + 2H+  I2 + 2H2O + 2K+ ----- equation 2 I2 + 2Na2S2O3  2NaI + Na2S4O6 ----- equation 3 You are to determine the concentration of hydrogen peroxide in R and use this to determine the relative formula mass of barium peroxide. Q is 0.100 mol/dm3 sodium thiosulfate. Read all the instructions below carefully before starting the experiments in Question 1. Instructions (a) Put Q into the burette. Pipette a 25.0 cm3 portion of R into a flask and add one full test-tube of dilute sulfuric acid followed by about one full test-tube of aqueous potassium iodide. The solution should turn red-brown. Do not add the starch indicator at this stage. Add Q from the burette until the red -brown colour fades to pale yellow, then add a few drops of the starch indicator. This will give a dark blue solution. Continue adding Q slowly from the burette until one drop of Q causes the blue colour to disappear, leaving a colourless solution. This is the end-point of the titration. Record your titration results in the space provided, repeating the