Copy of CHS 2024 S4 OP Prelim Paper 3 Answers

1 2024 Sec 4 Preliminary / Year 4 End-of-Year Examination Chemistry 6092/03 Practical Answer scheme Qn Type Answers 1(ai) MMO PDO [T] Table containing headings with correct units [1] • Final/initial (burette) reading / cm3 • Volume (of P) / cm3 I: units with values [R] Burette Readings recorded to nearest 0.05 cm3 + correct volume of P calculated [1] [A] Accuracy [2] Teacher value 20.60 cm3 for all shifts for average titre (of consistent readings) within 0.20 cm3 of SS’s average value score - 2 marks for corrected titres for average titre (of consistent readings) within 0.30 cm3 of SS’s average value score - 1 mark for corrected titres [C] 2 Consistent readings within 0.20 cm3 of each other [1] Example: Titration number 1 2 Final burette reading / cm3 20.60 20.60 Initial burette reading / cm3 0.00 0.00 Volume of P / cm3 20.60 20.60 Best titration results ✓ ✓ 1(aii) MMO calculates mean correctly to 2 decimal places (dp) [1] - candidate must take the average of two (or more) titre values that are within a total spread of not more than 0.20 cm3 / average of closest titre values - working / explanation must be shown or ticks must be put next to the two (or more) accurate readings selected - the mean should be quoted to 2 d.p. and be rounded to the nearest 0.01 cm3 1(bi) ACE Correctly calculates amount of HCl used [1] amount of HCl = 0.100 × volume in (a)(ii) 1000 (mol) 1(bii) ACE Correctly uses equation [1] amount of sodium carbonate = (b)(i) × 1 2 (mol) 1(biii) ACE concentration of sodium carbonate in 1.00 dm3 = amount of sodium carbonate from (b)(ii) × 1000 25.0 (mol/dm3) 1b(iv) ACE Mr= mass moles Mr= 14.30 concentration from (b)(iii) [1] ; Mr of nH2O = Mr – 106 n = 𝑀𝑟 𝑜𝑓 𝑛𝐻2𝑂 18 [1] leaving answers to 3sig fig for b(i), b(ii) and b(iii) [1];

2 1(c) ACE Volume of P to be smaller than expected + amount of P to be smaller than expected + amount of Na2CO3 to be smaller than expected ; n will be larger than expected; R: increase 1(d) MMO PDO precision of volume to .0 or .5 cm3 ; calculate mass of P correctly + precision of mass to 2 dp ; 1(ei) ACE Gas lost from the boiling tube before the rubber bung was replaced 1(eii) ACE use a small test-tube and thread/ drop(ping) funnel/ thistle funnel; R: delivery funnel/ dropper flask/ dropper funnel 2(a) MMO PDO black solid/residue; gas produced relights a glowing splint + oxygen (gas) is produced ; (A: glowing splint becomes brighter) 2(b)(i) MMO PDO 1 (green/dark green/blue-green solution) which turns purple/ dark purple/ black (upon standing) 2 purple solution 3 purple* solution turns colourless / pale yellow / yellow ; OR (pale) green solution turns pale/ light yellow / yellow *mention ‘purple’ at least once in Test 2 or 3 4 off-white ppt. (formed) + insoluble in excess (aq. ammonia) ; A: light yellow/ light brown/ cream/ beige / yellow/ brown R: white/ whitish 5 no observable changes + 6 white ppt. ; 2(b)(ii) ACE redox ; (pale yellow/yellow) Fe3+ / Fe2(SO4)3 is formed + oxidation state of Fe in FeSO4 increases from +2 to +3 OR Fe2+ loses electron (to form Fe3+) + hence Fe in FeSO4 is oxidised (by acidified KMnO4) ; A: oxidation 2(b)(iii) ACE SO42- / Sulfate + Test 6 ; 2(b)(iv) ACE Does not affect (conclusion) + any carbonate present will be removed by dilute nitric acid regardless of order of addition (OWTTE) ;

3 3(A) PDO [Axes] Axes labelled + units ; [Scale] Appropriate scale + every 10 small squares marked + plotted points take up more than 50% of graph grid [Plots] Plot all points correctly within half of smallest square [Line] Two best-fit straight lines ; 3(B) ACE Reading off the best-fit line with dotted line drawn + indicate value on y-axis OR coordinates written + y-value (0.25 oC) error allowance is half the smallest square ; 3(C) ACE As volume of B increases from 0 cm3 to 25 cm3, the highest temperature reached increased + As volume of B increases from 25 cm3 to 40 cm3, the highest temperature reached decreased ; As volume of B increased from 0 cm3 to 25 cm3, B is the limiting reactant. As volume of B increased from 25 cm3 to 40 cm3, all the alkali is neutralised/ reaction is complete (OWTTE), no more heat is produced (so heat is evenly distributed over a larger volume.) 4 P Method: Measuring mass over time 1. Measure a known/fixed volume of drink using a measuring cylinder / pipette / burette 2. Add excess (solid) NaHCO3 3. suitable apparatus for experiment: conical flask (R: beaker) + cotton wool placed in the mouth of the conical flask (to prevent acid spray, while allowing the carbon dioxide gas to escap e) + (electronic) mass balance + stopwatch OR labelled diagram 4. Measure mass of the conical flask and its contents at fixed time intervals (e.g. 1-min intervals) (until no change in mass) 5. Repeat experiment with the other fizzy drink 6. Plot a graph of the mass of the conical flask and its contents against time 7. for both fizzy drinks on the same axes / calculate (initial) gradients 8. The graph with a steeper (initial) gradient is the drink with a higher concentration of phosphoric acid

4 Category Specific details Tick Mark Quantity excess (solid) NaHCO3  known/ fixed volume of fizzy drink  Apparatus stopwatch  burette/ pipette/ measuring cylinder  conical flask (R: beaker)  mass (electronic) balance  cotton wool  Measurement mass of the conical flask and its contents at fixed time intervals  until no change in mass (R: until end of reaction/ reaction is complete, no more effervescence is seen)  Conclusion plot a graph of the mass of the conical flask and its contents against time  plot both graphs on the same axes/ calculate initial gradient  graph with steeper initial gradient is the one with higher concentration of phosphoric acid  Category Specific details Tick Mark Quantity excess (solid) NaHCO3  known/ fixed volume of fizzy drink  Apparatus stopwatch  burette/ pipette/ measuring cylinder  conical flask (R: beaker)  mass (electronic) balance  cotton wool  Measurement mass of the conical flask and its contents at fixed time intervals  until no change in mass (R: until end of reaction/ reaction is complete, no more effervescence is seen)  Conclusion plot a graph of the mass of the conical flask and its contents against time  plot both graphs on the same axes/ calculate initial gradient  graph with steeper initial gradient is the one with higher concentration of phosphoric acid  Category Specific details Tick Mark Quantity excess (solid) NaHCO3  known/ fixed volume of fizzy drink  Apparatus stopwatch  burette/ pipette/ measuring cylinder  conical flask (R: beaker)  mass (electronic) balance  cotton wool  Measurement mass of the conical flask and its contents at fixed time intervals  until no change in mass (R: until end of reaction/ reaction is complete, no more effervescence is seen)  Conclusion plot a graph of the mass of the conical flask and its contents against time  plot both graphs on the same axes/ calculate initial gradient  graph with steeper initial gradient is the one with higher concentration of phosphoric acid  Category Specific details Tick Mark Quantity excess (solid) NaHCO3  known/ fixed volume of fizzy drink  Apparatus stopwatch  burette/ pipette/ measuring cylinder  conical flask (R: beaker)  mass (electronic) balance  cotton wool  Measurement mass of the conical flask and its contents at fixed time intervals  until no change in mass (R: until end of reaction/ reaction is complete, no more effervescence is seen)  Conclusion plot a graph of the mass of the conical flask and its contents against time  plot both graphs on the same axes/ calculate