Copy of 2024 SCGS Chemistry Prelim P3 answers

11 CANDIDATE NAME ANSWERS 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.

12 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 titration as many times as you consider necessary to achieve consistent results. Presentation (indicate qn number): –1 overall Results Titration Number 1 2 Final burette reading / cm3 25.10 25.10 Initial burette reading / cm3 0.00 0.00 Volume of Q used / cm3 25.10 25.10 Best results ()   Headers with units (1), 2 d.p. for burette readings and (b) (1), accuracy (2) ±0.20 per reading, concordance (1) rej: vol, amount, final volume of Q

13 [5] (b) From your titration results, obtain an average volume of Q to be used in your calculations. Show clearly how you obtained this volume. Average volume = ଶହ.ଶ଴ ାଶହ.ଶ଴ ଶ = 25.20 cm3 working + answer average volume of Q …………………………. cm3 [1] (c) Q is 0.100 mol/dm3 sodium thiosulfate. Use equation 3 to calculate the number of moles of iodine liberated. No. of moles of Na2S2O3 = ଶହ.ଶ଴ ଵ଴଴଴ x 0.100 = 0.00252 mol (ecf) No. of moles of I2 = ଵ ଶ x 0.00252 = 0.00126 mol number of moles of iodine …………………………. mol [1] (d) Using your answer from (c) and equation 2, calculate the concentration in mol/dm3, of hydrogen peroxide in R. No. of moles of H2O2 in 25.0 cm3 of P = 0.00126 mol (ecf) Concentration of H2O2 in P = 0.00126 ÷ ଶହ ଵ଴଴଴ = 0.0504 mol/dm3 (ecf) No working – 0m concentration of hydrogen peroxide in R …………………………. mol/dm3 [2] (e) R was prepared by adding dilute sulfuric acid to 8.50 g of barium peroxide and diluting the resulting solution to 1.00 dm3. Using your answer to (d), calculate the relative formula mass of barium peroxide. Mr of BaO2 = 8.50 ÷ 0.0504 = 168.65 ≈ 169 (3 s.f.) 1m for relative formula mass (no units) 1m working Wrong working, correct answer – 0m relative formula mass of barium peroxide is …………………………. [2] 25.20 0.00126 0.0504 169

14 (f) After rinsing the burette with distilled water, a student did not rinse her burette with Q, before performing the titration. State and explain the impact of this error on her titration results. …………………………………………………………………………………………………… .. …………………………………………………………………………………………………… .. …………………………………………………………………………………………………… .. ………………………………………………………………………………………………… ..[2] (g) Using oxidation numbers, show that equation 2 is a redox reaction. …………………………………………………………………………………………………… .. …………………………………………………………………………………………………… .. …………………………………………………………………………………………………… .. ………………………………………………………………………………………………… ..[2] [Total: 15] As the concentration of Q is lowered [1] due to dilution, a larger volume of Q is required to deliver the same number of moles. As such, she will get a larger than expected volume of Q used. [1] accept: more dilute rej: affect results / amount of Q KI is oxidised as the oxidation number (O.N.) of iodine increases from –1 in KI to 0 in I2. [1] H2O2 is reduced as the O.N. of oxygen decreases from –1 in H2O2 to –2 in H2O. [1] Since both oxidation and reduction occurs together, this is a redox reaction.

15 2 (a) You are provided with solid C. Carry out the following tests and record your observations in the table. You should test and name any gas evolved. C is copper(II) carbonate test observations Test 1 Place 2 spatulas of solid C in a clean test -tube. Heat the test tube strongly. Green solid turns black. [1] CO2 gas [1] evolved gives a white ppt. in limewater. [1]. (credited once for Test 1 and Test 2) Test 2 Place half a spatula of solid C in a clean test-tube. Carefully add dilute sulfuric acid until the reaction is complete. Use a glass rod to stir the mixture. Keep the final solution for use in Test 3 – 4. Green solid dissolves/forms/reacts/disappears to form a blue solution. [1] Effervescence observed [1]. Test 3 Place 1 cm depth of the solution formed in Test 2 in a clean test tube. Add aqueous ammonia slowly with shaking, until no further change is seen. Light blue ppt. formed, soluble in excess to give a dark blue solution. [1] Test 4 Place 1 cm depth of the solution formed in Test 2 in a clean boiling tube. Add an equal volume of potassium iodide. Leave the mixture to stand for a few minutes to observe the colour(s) of the product(s) clearly. Keep the mixture for Test 5. (Ignore brown ppt. formed) White/grey/light-brown/beige/off-white ppt. [1] settled below the test -tube, with a brown solution above. [1] rej: cream/light yellow Test 5 To the mixture obtained in Test 4, add solution Q until the boiling tube is half-full. (BOD/ecf from test 4) Brown solution turns colourless. [1] Ignore white ppt. Rej: brown solution turns yellow. [9]

16 (b) You are provided with solution D. Carry out the following tests and record your observations in the table. You should test and name any gas evolved. D