SNGS 2024 Chem Prelim Paper 3 Mark Scheme

2024 S4 Chem Prelim Practical Examination Answer and Mark Scheme Question 1 (a) PDO MMO Results table Titration number 1 2 Final reading / cm3 Initial reading / cm3 Volume of P used / cm3 18.60 18.40 Best titration results Format of table can be different but the main quantities must be in the table Average reading: cm3 If ticking of best results not shown in table, penalise under header Record initial burette reading, final burette reading and volume added with correct headings and units in a titration table. all burette readings for all accurate titres in titration table are recorded to nearest 0.05 cm3. Volume of P calculated correctly. Titration results Accuracy • For average titre (of consistent readings) within 0.20 cm3 of supervisor’s average value score 2 marks ( • For average titre (of consistent readings) within 0.30 cm3 of supervisor’s average value score 1 mark Concordance • At least two titre values are within 0.20 cm3 (using uncorrected titres) [1] [1] [2] [1] (b) MMO Appropriate average volume of P in 2 d.p. from closest titre values (identified by a tick or in calculation) If value is 18.28 cm3 but round up to 18.30 cm3 at the end penalise as dp [1] (c) ACE (average volume × 0.100) ÷ 1000 to 3sf [1] (d) ACE moles of citric acid = (c) / 3 [1] (e) ACE concentration of citric acid = (d) × 1000 / 25 [1]

(f) ACE mass of citric acid = (d) × 20 x 192 or [(e) / 2]x192 Appropriate decimal place, significant figures in final answer of b), c), d), e), f). [1] Appropriate units in final answer of b), e), f) [units must be on the line] [1] (for final ans, if working is correct, will still get full marks as per qns, final ans need not be on line) [3] (g) ACE 1/3 of part (b) (must give specific value, ignore dp/sf) [1] Ethanoic acid is monobasic while citric acid is tribasic. [1] (accept similar explanations) The mole ratio of KOH to ethanoic acid is 1:1 while the mole ratio of KOH to citric acid is 3:1. [1] (accept similar explanations) If show by balanced eqns only, give 2 out of 3 [3] Total: 15

Question 2 (a) MMO test observations Test 1 Put about 1 cm depth of aqueous hydrogen peroxide in a clean test tube. Add an equal volume of dilute sulfuric acid. Then add 10 drops of T with shaking. You do not need to test any gas evolved in Test 1. Purple solution turned colourless [1] Effervescence, colourless, odourless gas evolved. [1] (penalise once if use bubbling/bubbles and if colourless/odourless does not appear) Test 2 Put about 1cm depth of hydrogen peroxide in a clean test tube. Use a spatula to carefully add a small amount of D Effervescence Colourless, odourless gas evolved. [1] Gas relights a glowing splint. Gas is oxygen [1] Test 3 Put about 1 cm depth of dilute sulfuric acid in a clean test tube. Add an equal vo