Copy of AMKSS 2024 Chem P3 Answers

1 ANG MO KIO SECONDARY SCHOOL PRELIM EXAMINATION 2024 SECONDARY FOUR EXPRESS CHEMISTRY [6092/03] ANSWER SCHEME Paper 3: Chemistry Practical [40 Marks] Note: Overall deduct 1 mark if no 3 sf. Note: Overall deduct 1 mark if no unit. 1(a) Results Titration Number 1 2 3 Final burette reading / cm3 23.60 [shift 1] 23.80 [shift 2] 23.60 [shift 1] 23.80 [shift 2] Initial burette reading / cm3 0.00 0.00 Volume of potassium manganate(VII) used / cm3 23.60 [shift 1] 23.80 [shift 2] 23.60 [shift 1] 23.80 [shift 2] Best titration results (√) √ √ • Correct headings with units of measurement [1] • All burette readings for all accurate titres in titration table are recorded to nearest 0.05 cm3 [1] • Accuracy of titration results [2] - for the average titre within 0.20 cm3 of Supervisor’s average value score 2 marks - for the average titre within 0.30 cm3 of Supervisor’s average value score 1 mark • At least two titre values are within 0.20 cm3 [1] 5 1(b) Average volume of G = (23.60 + 23.60) / 2 = 23.60 cm3 1 1(c) Number of moles of H = 0.08 mol/dm3 x (25 / 1000) = 0.00200 mol ( 3sf ) 1 1(d) manganate(VII) ions : iron(II) ions MnO4- : Fe2+ 1 : 5 Moles of manganate(VII) ions = 0.002 ÷ 5 = 0.0004 [1] number of moles of manganate(VII) ions in 1 dm3 = 0.0004 ÷ (23.60/1000) = 0.0169491 = 0.0169 mol [1] Note: E.C.F allowed if working is correct. ( if 1c is wrong ) 2 1(e) Number of moles of manganate(VII) ions in 250 cm3 of G = 0.0169 ÷ 4 = 0.004225 mol = 0.00423 mol 1

2 Note: E.C.F allowed if working is correct. ( if 1d is wrong ) 1(f) Mr of Manganese = 55 Mass of manganese in 2.12g = 55 x 0.00423 = 0.23265 g = 0.233 g Note: E.C.F allowed if working is correct. ( if 1e is wrong ) 1 1(g) percentage by mass of manganese in mangalloy = (0.233 / 2.12) x 100% = 10.99% = 11.0% [1] Note: E.C.F allowed if working is correct. ( if 1f is wrong ) 1 1(h) Percentage mass will be higher. [1] The number of moles of G will be higher as concentration is divided by a lower volume. 1 2(a) total volume of Q added /cm3 highest temperature reached/°C [shift 1] highest temperature reached/°C [shift 2] 0.00 31.5 31.5 5.00 33.5 33.5 10.00 35.0 35.5 15.00 36.0 36.5 20.00 37.0 37.5 25.00 36.5 37.0 30.00 36.0 36.5 35.00 35.0 36.0 40.00 34.5 35.5 • All temperature readings are recorded to nearest 0.5 °C [1] • All data completed with correct trend [1] Note: As vol. of Q increases, temperature increases, As Q = 20.00 cm3, temperature is the highest. As vol. of Q continues to increases, temperature decreases. Minus 1 mark if there are 3 or more constant temperatures for the upward/downward trends respectively. 2 2(b) Plotting of graph • Labelled axes with units of measurement and appropriate scale for axes [1] • All points plotted accurately [1] • 2 Best-fit straight lines (intersect) [1] Note: wrong scale if students starts from 0Oc for the y axis. 3 2(c) Temperature change increases until neutralisation [1] then decreases as more Q is added because reaction has stopped. [1] Marker’s: remarks: 1) Most students did not read explain the trend. They only describe the trend. 2

3 2(d) values from graph with units and indication shown Rejected: if not indicated on the graph. Check for 2dp. ( overall minus one mark ) 1 2(e) Experimental error: heat loss to the surroundings Impact: temperature recorded may be lower than actual 1 1 3(a) Gas evolved turns damp/moist red litmus paper blue. (✓) Gas evolved is ammonia. (✓) Reject: Effervescence White precipitate formed when gas is bubbled through limewater. (✓) Gas evolved is carbon dioxide. (✓) Solid changes from white to yellow on heating. (✓) Reject: orange/ reddish-brown on heating – ( when X not heated strongly ) Solid changes from yellow (ecf colour from above) to white/ pale orange/ orange-white/ pinkish white/ pale pink on cooling/ after some time. (✓) Reject: pale yellow – solid has not cooled sufficiently Water droplets seen at the mouth of the test tube. (✓) 5✓ [4] ; 4✓ [3] ; 3✓ [2] ; 2 ✓ [1] (students need to identify at least one gas to get full marks) Marker’s: remarks: 1) many students did not read instructions that they have to identify the gases. 2) many students did not read instructions that they have to identify the gases. 3) Many students test positive for hydrogen gas. 4) Many students did not write ‘damp / moist “ for red litmus. 4 3(b) Effervescence. (✓) Gas evolved produced forms white precipitate with limewater. (✓) Gas evolved is carbon dioxide. (✓) Solid dissolves to form colourless solution / solid partially dissolves to form colourless solution (✓) Effervescence wrong spelling – penalise once. 4✓ [2] ; 2 – 3✓ [1] Rejected: illegible handwritings, esp on the word” effervescence “. Marker’s: report : 1) Many students test positive for hydrogen gas. 2) Many students scribble “ effervescence “. Strictly no mark awarded 2 3(c) White precipitate forms, (reject solid) (✓) Ppt dissolves in excess NaOH (aq) to form colourless solution. (✓) Gas evolved on warming, turns damp/ moist red litmus paper blue. (✓) Gas evolved is ammonia. (✓) 2

4 4✓ [2] ; 2 – 3✓ [1] Marker’s: remarks 1) Majority of the students did not use QA list and give incomplete answers. 2) Half of the students claim ppt. is insoluble in excess. 3) Many students did not write ‘damp / moist “ for red litmus. 3(d) White precipitate forms, (✓) ppt dissolves in excess NH3 (aq) to form colourless solution (✓) 2 ✓ [1] Marker’s: remarks: 1) Majority of the students did not use QA list and give incomplete answers. 2) Half of the students claim ppt. is insoluble in excess. 1 3(e) No precipitate formed / No observable change [1] Reject: White precipitate, soluble A: No visible change / solution remains colourless. Marker’s: remarks: 1) Accept: No visible change. / No change is observed Reject: No Change 1 3(f) NH4+, Zn2+, CO32- [1] 1 4 heating to dryness method max [6]: - - weigh (any) sample of washing soda (let mass be a) - - heat (to remove water of crystallisation) in evaporating dish/crucible - - let cool and reweigh - - repeat heating - to constant mass (let mass be b) - appropriate calculation suggested for the percentage of water: (a-b)/a x 100% OR mass of water method max [6]: - - weigh (any) sample of washing soda (let mass be a) - - heat to remove water of crystallisation in round bottom flask in a simple distillation set up - cool / condense water vapour in condenser - continue until no more water collected as distillate - weigh mass of water collected (let mass be b) - appropriate calculation suggested for the percentage of water b/a x 100% Wrong method zero mark. : Add water / reacts acid etc/ 1 1 1 1 1 1 1 1 1 1 1 1

5 Paper 3: Practical Preparation List Label per candidate identity notes H 150 cm3 H is a solution which initially contained 0.0800 mol/dm3 iron(II) ions dissolved in an unknown acid freshly prepared solution) dissolve 27.0 g of hydrated iron (II) sulfate, FeSO4.7H2O in 1 dm3 of approximately 0.5 mol/dm3 sulfuric acid, labelled H G 150 cm3 0.020 mol/dm3 potassium manganate (VII) Solution of 0.020 mol/dm3 potassium manganate (VII) (3.2 g KMnO4 dissolved in 1 dm3 of distilled water), labelled G. P 150 cm3 1.00 mol/dm3 hydrochloric acid Q 100 cm3 1.2 mol/dm3 aqueous sodium hydroxide R 2.00g mixture of two carbonates: (NH4)2CO3 ZnCO3 0.6 g 1.4 g Place this mixture of carbonates in a small plastic vial. Note: Some variation in the above concentrations is acceptable but it is essential that 25.0 cm3 H reacts with