SAJC 2024 Prelim P4 Answers

1 Name: Class: ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION CHEMISTRY Paper 4 Practical 9729/04 15 August 2024 2 hours 30 minutes Additional Materials: Qualitative Analysis Notes Shift Laboratory For Examiner’s Use 1 2 3 4 Total This document consists of 18 printed pages including this page. READ THESE INSTRUCTIONS FIRST. Write your name and class on all the work you hand in. Give details of the practical shift and laboratory in the boxes provided above. Write in dark blue or black pen. You may use a soft pencil for any diagrams or 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. The number of marks is given in the brackets [ ] at the end of each question or part question. 13 17 15 10 55

2 Answer all the questions in the spaces provided. 1 Identification of unknown cation, anion and acids FA 1 is a salt which contains a cation and an anion. FA 2 is an aqueous mixture of two acids. The possible acids are hydrochloric acid, nitric acid and sulfuric acid. You will perform tests to identify the ions in FA 1 and determine which acids are present in FA 2. Test and identify any gases evolved, unless otherwise stated. If there is no observable change, write no observable change. (a) (i) Carry out the following tests and record your observations in Table 1.1. Table 1.1 tests observations 1 Add approximately one spatula of FA 1 in a boiling tube and heat strongly for about 1 minute, then leave to cool. Do not test for gas. White solid turned yellow upon strong heating. Yellow solid turned white again upon cooling. Note: Some students might have also observed colourless droplets forming on the top of the boiling tube. 2 To an approximately 2 cm depth of FA 2 in a test tube, carefully add approximately one spatula of FA 1. Keep this solution for use in 1(b)(i). White solid dissolved to form colourless solution. Effervescence observed. Gas forms white ppt with limewater. Gas is CO2. [4]

3 (ii) Identify the anion in FA 1 and write an ionic equation to explain how you deduced its identity, using your observations in Table 1.1. …………………………………………………………………………………………… …………………………………………………………………………………………… [1] Anion is CO32− 2H++ CO32− → CO2 + H2O (b) (i) Devise and perform a series of simple tests on the solution from Test 2 in (a)(i) to identify the cation in FA 1. Your tests should be based on the Qualitative Analysis Notes on pages 17 to 18 and should only use the bench reagents provided. Record your tests and observations in the space below. Test Observations To 1 cm3 of solution from Test 2 in a test tube, add NaOH(aq) dropwise until in excess. White ppt formed which was soluble in excess NaOH. To 1 cm3 of solution from Test 2 in a test tube, add NH3(aq) dropwise until in excess. White ppt formed which was soluble in excess NH3. [2]

4 (ii) Use your observations in (b)(i) to identify the cation in FA 1. Zn2+ (ecf from observations) …..………………………………………………………………………………… [1] (c) (i) Devise and perform a series of simple tests to identify the acids present in FA 2. Your tests should be based on the Qualitative Analysis Notes on pages 17 to 18 and should only use the bench reagents provided. Record your tests and observations in the space below. Any test requiring heating MUST be performed in a boiling tube. Tests Observations To 1 cm3 of FA 2 in a test tube, add AgNO3(aq), followed by NH3(aq). White ppt formed which was soluble in NH3 to form colourless solution. To 1 cm3 of FA 2 in a boiling tube, add 1 cm3 of NaOH(aq) and place 1 piece of Al foil inside the boiling tube. Heat gently. Gas evolved turns damp red litmus paper blue. Note: Students need to add sufficient NaOH(aq) in order to produce ammonia gas. To 1 cm3 of FA 2 in a test tube, add Ba(NO3)2(aq). No ppt formed. [4] (ii) State the formulae of the acids present in FA 2. HCl and HNO3 …………………………………………………….. [1] [Total: 13]

5 2 Determination of the identity of a straight-chain carboxylic acid Compound E is a straight-chain monobasic carboxylic acid. It is reacted with excess alkali and the unreacted alkali is then titrated with an acid. FA 3 was prepared as follows: • 200 cm3 of 0.30 mol dm−3 aqueous sodium hydroxide was transferred into a beaker. • 2.5 g of E was added. • The solution was heated for 1 minute to ensure complete reaction. To determine the identity of compound E, you will first perform a dilution of hydrochloric acid, FA 4, and titrate this with FA 3. FA 4 is 2.00 mol dm−3 hydrochloric acid, HCl. You will also use FA 4 for Question 3. Solution S is screened methyl orange indicator. (a) (i) Dilution of FA 4 1. Use a pipette to transfer 25.0 cm3 of FA 4 into a 250 cm3 volumetric flask. 2. Make up the contents in the volumetric flask to the mark with deionised water. 3. Stopper the flask and mix thoroughly to obtain a homogeneous solution. Label this diluted acid as FA 5. 4. Wash the pipette thoroughly before moving on to the next set of instructions for titration. 5. Do not discard FA 4. Keep the remaining FA 4 solution for Question 3. (ii) Titration of FA 3 against FA 5 using solution S 1. Fill the burette with FA 5. 2. Use a pipette to transfer 25.0 cm3 of FA 3 into a conical flask. 3. Add a few drops of solution S into the conical flask. 4. Run FA 5 from the burette into the conical flask. The end -point is reached when the solution changes from green to grey. If the solution becomes violet, you have passed the end-point. 5. Record your titration results to an appropriate level of precision in the space on the next page. 6. Repeat steps 2 to 5 until consistent results are obtained. Titration results Titration number 1 2 Final burette reading / cm3 20.25 40.50 Initial burette reading / cm3 0.00 20.25 Volume of FA 5 / HCl used / cm3 20.25 20.25 [2]

6 (iii) From your titrations, obtain a suitable volume of FA 5 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 5 used = 20.25 + 20.25 2 = 20.25 cm3 volume of FA 5 = ……………………. [3] (b) (i) Calculate the amount, in moles, of sodium hydroxide added to compound E during the preparation of FA 3. Initial amount of NaOH = 0.200 x 0.30 = 0.0600 mol [1] (ii) Use your titration results to calculate the amount, in moles, of sodium hydroxide in the beaker after reaction with compound E during the preparation of FA 3. Hence, calculate the amount, in moles, of sodium hydroxide that reacted with compound E. [HCl] in FA 5 = 2.00 / 10 = 0.200 mol dm−3 Amount of HCl used in titration = 0.200 x 0.02025 = 0.00405 mol = Amount of NaOH in 25.0 cm3 of FA 3 Amount of unreacted NaOH in 200.0 cm3 of FA 3 (beaker) = 0.00405 x (200/25) = 0.0324 mol Amount of NaOH that reacted with E = 0.06 – 0.0324 = 0.0276 mol (ecf) [3] (iii) Use your answer to (b)(ii) to calculate the Mr of E. Record your answer to 1 decimal place. Hence, deduce the identity of E, showing your working clearly. [Ar: C, 12.0; H, 1.0; O, 16.0] Amount of E = 0.0276 mol Mr of E = 2.5 / 0.0276 (ecf) = 90.6 (1 d.p.) Mr of E = ……………………. Mr of COOH = 45 Mr of CxH2x+1 = 45.6 (ecf) x = 3 (approx.) E is butanoic acid (C3H7COOH) (must be straight-chain COOH) Identity of E = …………….……