MI Prelim H2 Chem P4 Suggested answers_for exchange

Class Adm No Candidate Name: 2025 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/04 Paper 4 Practical 28th Aug 2025 2 h 30 min Suggested answers Shift Laboratory H

2 1 Identification of anions FA 1 is an aqueous solution containing two anions. You will perform tests to identify the two anions in FA 1. You are not expected to identify the cations. Unless otherwise stated, the volumes given below are approximate and should be estimated rather than measured. Test and identify any gases evolved. For Examiners’ Use (a) Carry out the following tests. Carefully record your observations in Table 1.1. Table 1.1 tests observations 1 Test FA 1 solution using Universal Indicator paper. Universal Indicator paper turns blue/dark blue/dark violet (pH 14) 2 To a 1 cm depth of FA 1 in a clean test- tube, add 1 cm depth of aqueous silver nitrate. Add excess aqueous ammonia. Brown ppt forms Ppt dissolves in excess aqueous ammonia to give a colourless solution 3 To a 1 cm depth of FA 1 in a clean boiling tube, add a small piece of aluminium foil into the boiling tube, then heat gently over a Bunsen flame. Gas evolved upon warming turns damp red litmus blue. Gas is NH3. 4 To a 1 cm depth of FA 1 in a clean test- tube, add 2 cm depth of dilute hydrochloric acid. No effervescence observed [4]

3 [Turn over (b) Identify the two anions in FA 1 and state the evidence for each anion by completing Table 1.2. The evidence given must be sufficiently conclusive in identifying the anions. Table 1.2 anion evidence OH– Test 1: Solution is alkaline, indicating presence of OH– ions and Test 2: Solution contains OH− which forms brown ppt (Ag2O) when AgNO3 is added OR Test 3: NH3 gas is evolved when heated with Al, hence OH− ions are must be present NO3– Test 3: NH3 gas evolved indicates presence of NO3– or NO2– and Test 4: brown NO2 gas not evolved, indicating absence of NO2– [4] [Total: 8]

4 2 Investigation on a solution of oxalic acid Oxalic acid is a common organic acid containing the carboxylic acid functional group. You will perform two separate experiments to investigate its properties. For Examiners’ Use (a) Determination of concentration and enthalpy change of neutralisation by thermometric titration Oxalic acid is an organic acid that occurs naturally in many foods, such as quinoa and rhubarb leaves. It is known to be a dibasic acid, potentially donating two H+ ions per molecule of oxalic acid. FA 2 is aqueous oxalic acid of unknown concentration. FA 3 is 2.2 mol dm-3 sodium hydroxide, NaOH. Prepare a table in the space provided on the next page and record, to the appropriate level of precision: • all volumes of FA 3 added,