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 volume of aqueous potassium iodide. Then add a small amount of D . Pale/light yellow/colourless solution turns –reddish brown/Brown [1] Test 4 Put about 1 cm depth of S in a clean test tube. Add aqueous sodium hydroxide until a change is seen. Add excess aqueous sodium hydroxide to the mixture and allow to stand for a few minutes. (Light) cream/beige/brown/yellow/ orange ppt insoluble in excess aqueous NaOH.[1] Test 5 Put about 1 cm depth of S in a clean test tube. Add an equal volume of aqueous barium nitrate then add an equal volume of dilute nitric acid. White ppt insoluble in aqueous nitric acid/no visible change [1] Test 6 Put about 1 cm depth of T in a clean test tube. Add an equal volume of S Purple solution turns brown/colourless[1] brown ppt observed[1]

Note: D MnO2 T KMnO4 S MnSO4 (b) ACE D is oxidising agent. [1] The oxidation state of iodine increased from -1 in KI to 0 in I2. [1] OR KI becomes I2 (reject KI turns brown) [2] Total: 11 Question 3 (a) PDO Labelled x and y axis with units correctly [1] Appropriate scale and graph is at least ¾ of graph paper [1] Plotted all points correctly [1] Appropriate best fit line drawn [1] [4] (b)(i) PDO Volume of acidified iron(II) sulfate is _______27.5___________ cm3 (dp to half of the smallest sq) [1] (ii) PDO Volume of acidified hydrogen peroxide is ___22.5_________ cm3. [1]

(c)(i) ACE Mole of iron(II) sulfate = 0.05x0.0275 =0.001375 =0.00138mol [1] (ii) ACE Mole of hydrogen peroxide = 0.03x0.0225 =0.000675mol Penalise (c)(i) & (ii) once if never convert to dm3 [1] (d) ACE FeSO4 : H2O2 0.00138 : 000675 0.00138/000675 : 0.000675/0.000675 2 : 1 ____2___ FeSO4 + ____1__H 2O2 + sulfuric acid → iron(III) sulfate + water (accept the correct rounded off values of what they have) [1] Total marks: 9 Question 4 P To a fixed/known mass of A, [if wrong reagent, can only get 1 m for this] add excess aqueous sodium hydroxide / aq ammonia (but avoid using weak alkali) to separate Filter the mixture to obtain magnesium oxide as residue. residue magnesium oxide is washed, dried and its mass is measured. % purity of magnesium oxide = mass of residue/mass of A x 100 [1] [1] [1] [1] [1] Total marks: 5