Christchurch 4E Prelim Chemistry 6092 P3 Final 2025

CHR 2025/Chemistry 6092/03/O/25 [Turn over CHRIST CHURCH SECONDARY SCHOOL 2025 PRELIMINARY EXAMINATION FOUR EXPRESS CHEMISTRY 6092/03 Paper 3 Practical 28 July 2025 1 hour 50 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the confidential instructions READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and class in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams, graphs. Do not use staples, paper clips, highlighters, 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 12. 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. _____________________________________________________________________ This document consists of 11 printed pages and 1 blank page. CANDIDATE NAME CLASS CENTRE NUMBER S INDEX NUMBER For Examiner’s Use 1 2 3 Total 40 Shift Laboratory

2 CHR 2025/Chemistry 6092/03/O/25 1 An oxyacid of phosphorus has the formula, H3PO3. You are required to find, by experiment, the number of moles of sodium hydroxide that react with 1 mole of this acid. P is 0.0984 mol/dm3 sodium hydroxide. Q is a solution containing 5.04 g of the oxyacid of phosphorus, H3PO3, in 1 dm3 of solution. Read all the instructions carefully before starting the experiments in Question 1. Instructions (a) (i) Fill the burette with solution Q. Pipette 25.0 cm3 of P into a flask and titrate with Q, using the indicator provided. Record your titration results in the space provided below, repeating the titration as many times as you consider necessary to achieve consistent results. Results [5] (ii) 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 of Q ……………………….. [1]

3 CHR 2025/Chemistry 6092/03/O/25 [Turn over (b) (i) P is 0.0984 mol/dm3 sodium hydroxide. Calculate the number of moles of sodium hydroxide in 25.0 cm3 of P. no. of moles of sodium hydroxide in 25.0 cm3 of P …………………….. [1] (ii) Q is an aqueous solution of H3PO3 with a concentration of 5.04 g/dm3. Calculate the molar concentration, in mol/dm3, of H3PO3 in Q. [Ar: H, 1; P, 31; O, 16] concentration of H3PO3 in Q …………………….. [2] (iii) Hence, using your answer from (b)(i) and (b)(ii), calculate the number of moles of sodium hydroxide which react with 1 mole of H3PO3. no. of moles of sodium hydroxide …………………….. [2] (iv) Using your answer to (b)(iii), write an equation for the reaction of the oxyacid of phosphorus, H3PO3, with sodium hydroxide. ……………………………………………………………………….……………….. [1]

4 CHR 2025/Chemistry 6092/03/O/25 (c) Using your results in (a)(ii), describe briefly how a pure and dry sample of the salt formed from the reaction of the oxyacid of phosphorus, H3PO3, with sodium hydroxide can be obtained. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. [3] (d) A student repeated the experiment. When preparing solution Q, the student accidentally measured 5.04 g of the oxyacid of sulfur, H2SO3, instead and dissolved it in 1 dm3 of solution. The number of moles of sodium hydroxide which react with 1 mole of H2SO3 is the same as the number of moles of sodium hydroxide which react with 1 mole of H3PO3. Describe and explain the effect this would have on the titration results recorded in (a)(i). [Ar: H, 1; S, 32; O, 16] …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. …….………………………………………………………………………………………….. [2] [Total: 17]

5 CHR 2025/Chemistry 6092/03/O/25 [Turn over 2 You are provided with two solids, R and S. You will carry out the following tests on the two solids, recording your observations at each stage. You should test and identify any gases evolved. Read the instructions carefully before starting the experiments. (a) Tests on Solid R test observations (i) Using a spatula, place approximately half of the solid R sample in a boiling tube. With a measuring cylinder, add about 8 cm3 of dilute sulfuric acid to the boiling tube. [2] Filter the reaction mixture from (a)(i). The filtrate is solution T. (ii) Pour about 1 cm depth of solution T into a test-tube. Add aqueous sodium hydroxide dropwise and then in excess. [2] (iii) Place the remaining solid R in a hard-glass boiling tube. Heat the solid strongly until no further change is seen. Then, stop the heating and leave the solid to cool. [2]

6 CHR 2025/Chemistry 6092/03/O/25 (iv) Identify the gas given off in (a)(i). …………………………………………………………………………………………… [1] (v) Explain why it is not possible to identify the cation contained in solution T from your observations in (a)(ii). …………………………………………………………………………………………… …………………………………………………………………………………………… [1] (vi) Suggest an additional test that can be done on solution T that would allow you to identify the cation contained in solution …………………………………………………………………………………………… …………………………………………………………………………………………… [1] (b) Tests on Solid S Place solid S in a boiling tube. With a measuring cylinder, add about 15 cm3 of distilled water to the boiling tube. Stopper the boiling tube and shake to dissolve the solid to form solution S. Divide this volume of solution S into three approximately equal portions in three test- tubes. test observations (i) Add the first portion of solution S to the aqueous chlorine provided in a test-tube. [1] (ii) To the second portion of solution S, add about 1 cm depth of dilute nitric acid followed by a few drops of aqueous silver nitrate. [1]

7 CHR 2025/Chemistry 6092/03/O/25 [Turn over (iii) To the third portion of solution S, add aqueous sodium hydroxide dropwise and then in excess. [2] (iv) Use evidence from the observations above to identify solid S. Identity: …………………………………………………………………………………. Evidence: ………………………………………………………………………………. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [3] [Total: 16]

8 CHR 2025/Chemistry 6092/03/O/25 BLANK PAGE

9 CHR 2025/Chemistry 6092/03/O/25 [Turn over 3 (a) You are provided with a rusty iron nail. The mass of the nail is between 0.10 to 0.20 g. Outline a method you would use to determine the mass of rust in the iron nail. You can assume all the apparatus and