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 c