Copy of 2024 CCHM Chemistry 6092 P3

[Turn over Name: Class: Class Register Number: PRELIMINARY EXAMINATION 2024 SECONDARY 4 CHEMISTRY 6092/03 Paper 3 Practical August 2024 1 hour 50 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and register number clearly in the spaces provided at the top of this page. 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 fluid. Answer all questions in the spaces provided on 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 10. 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. Shift Laboratory For Examiner’s Use 1 2 3 Total / 40 This document consists of 10 printed pages and 2 blank pages.

2 2024 Preliminary Exam/CCHMS/Secondary 4/Chemistry/6092/03 1 Magnesium oxide is often used to treat hear tburn. To determine the percentage purity of a sample of magnesium oxide, a method known as back titration is used. Procedure 1) Dissolve a sample of magnesium oxide in a known volume and concentration of hydrochloric acid, which is used in excess. MgO + 2HCl  MgCl2 + H2O 2) Determine the amount of unreacted acid by titrating the solution with aqueous sodium hydroxide. NaOH + HCl  NaCl + H2O 3) From the amount of unreacted acid, determine the amount of hydrochloric acid that has reacted with magnesium oxide. 4) Determine the amount of magnesium oxide that has reacted and hence its mass. Read all the instructions below carefully before starting the experiment in Question 1. P is aqueous 0.200 mol/dm3 sodium hydroxide. Q is a solution made by dissolving 1.20 g of magnesium oxide in 100 cm3 of 1.00 mol/dm3 dilute hydrochloric acid and the solution made up to 250 cm3 with distilled water. R is methyl orange indicator. (a) (i) Put P into the burette. Pipette 25.0 cm3 of Q into a conical flask. Add a few drops of R to the flask. Add P from the burette to the flask until the solution changes colour. Record your titration results in an appropriate format in the space provided. Repeat the titration as many times as you consider necessary to achieve consistent results. Results [5]

3 [Turn over 2024 Preliminary Exam/CCHMS/Secondary 4/Chemistry/6092/03 (ii) Calculate the average volume of P used. average volume of P = …………………... [1]