Copy of 2024 CCHM Chemistry 6092 P3 (MS)

1 2024 Sec 4 Prelim Chemistry P3 Mark Scheme Question Skill Indicative material Mark Total 1(a)(i) PDO MMO Results table: records initial burette readings, final burette reading and volume of Q added with correct headings and units in titration table. all burette readings for accurate titre in titration table recorded to nearest 0.05 cm3 Titration Results: 22.50 cm3 accuracy for the average titre (of consistent readings) within 0.20 cm3 of teacher’s average value scores 2 marks for the average titre (of consistent readings) within 0.30 cm3 of teacher’s average value scores 1 mark concordance at least two titre values are within 0.20 cm3 (using uncorrected titres) 1 1 2 1 [5] 1(a)(ii) MMO appropriate average volume of P in 2 d.p. from closest titre values (titre should be identified either in the table by a tick or in calculation) 1 [1] 1(b)(i) ACE No. of moles of P = 22.50/1000 x 0.2 = 0.00450 mol 1 mole of P reacts with 1 mole of HCl No. of moles of HCl = 0.00450 mol 1 [1] (ii) ACE No. of moles of HCl = 0.00456/0.025 x 0.25 (Allow e.c.f.) = 0.0450 mol 1 [1] 1(c)(i) ACE No. of moles of HCl that has reacted with MgO = (0.1 x 1 mol/dm3) – 0.0450 = 0.055 mol (Allow e.c.f.) 1 mole of MgO reacts with 2 moles of HCl No. of moles of MgO = 0.055/2 = 0.0275 mol 1 1 [2] (ii) ACE PDO Mass of MgO = 0.0275 x (24+16) (Allow e.c.f.) = 1.10 g (with unit) 1 [1]

2 1(c)(iii) ACE Percentage purity = 1.10/1.2 (Allow e.c.f.) = 91.7% 1 [1] (d) P Impurities in magnesium oxide does not react with acid. 1 [1] (e) ACE Water dilutes aqueous sodium hydroxide, lower concentration of sodium hydroxide. Higher volume of aqueous sodium hydroxide required to neutralize acid. 1 1 [2] Total: 15 Question Skill Indicative material Mark Total 2(a) MMO Effervescence. Gas evolved produces white precipitate in limewater. Gas is CO2 1 1 1 [3] 2(b) MMO No observable change. Gas evolved on warming, turns damp red litmus blue. Gas is NH3 1 1 1 [3] 2(c) MMO White precipitate formed and then dissolves / no ppt /no observable change effervescence on addition of nitric acid 1 1 [2] 2(d) MMO White precipitate formed Precipitate insoluble in HNO3 1 1 [2] 2(e) ACE NH4+/ammonium – When X is warmed with NaOH(aq) in (b), ammonia gas is produced CO32–/carbonate – When HCl (aq) added to X in (a), CO2 produced SO42–/sulfate – white precipitate formed with Ba(NO3)2 in (d), insoluble in HNO3 Na+/K+/sodium/potassium - the carbonate is soluble 1m each Alternative: identify NH4+, CO32–, SO42– 1m 1m each for each supporting evidence [4] Total: 14

3 Question Skill Indicative material Mark Total 3(a) P Stopwatch Gas syringe/burette 1 1 [2] 3(b) PDO Appropriate scale with correctly labelled axis