2024 Kranji Sec Pure Chem ANS

Suggested Answers for 2024 4Exp Prelim 6092 Chemistry P3 Q No. Answer Marks 1ai records initial burette reading, final burette reading, volume of P added with correct headings & units in titration table, all burette readings recorded to nearest 0.05 cm3, at least 2 titre values within 0.20 cm3, (Do not award marks for wrong calculations)* average titre of consistent readings with 0.20 cm3 of Supervisor’s average value [Award 1 m if average titre of consistent readings with 0.30 cm3 of Supervisor’s average value] (Do not award marks for accuracy if the concordance is not within 0.20 cm3) Titration No. 1 2 Final Burette Reading / cm3 22.70 22.70 Initial Burette Reading / cm3 0.00 0.00 Volume of P Used / cm3 22.70 22.70 Best Results √ √ Markers’ Comment: This standard titration table was surprisingly badly done. . 1 1 1 2 1aii Average volume (based on identified values) correctly calculated to 2 d.p. [Allow ECF from part 1ai]* 1 1bi No. of moles of thiosulfate ions= 0.120mol/dm 3 x (23.25 /1000) dm3 = 0.00279 mol (3 s.f) 1 1bii Compare mole ratio S 2O32- to Cu2+ No. of moles of S2O32- : No. of moles of Cu2+ 1:1 No. of moles of Cu2+ in 25 cm3 = 0.00279 mol No. of moles of Cu2+ in 1 dm3 = 0.00279 x (1000/25) = 0.1116 mol = 0.112 mol (3 s.f) [Allow ECF] 2 1biii Mass = no of moles of Cu x Ar of Cu = 0.1116 mol x 64 = 7.1424 g = 7.14g [Allow ECF] 1 1biv %mass of Cu = 7.1424/9.50 x 100%= 75.2% [1M] [1M ̶ unit (for answers in Q2aii and 2biii)] [1M ̶ 3 sig fig (for answers in Q2bi, ii, iii and iv)] 3 1c If potassium iodide is not in excess, not all Cu 2+ will be reacted. Less I2 will be produced. [1M] The volume of Na2S2O3 used will be lower which leads to a smaller mass of copper calculated and smaller percentage by mass of copper. [1M] 2 2a test observations Test 1 Put about 1 cm depth of hydrogen peroxide in a clean test-tube. Add an equal volume of dilute sulfuric acid. Then add 10 drops of C with shaking. You do not need to test any gas evolved in Test 1. Purple acidified potassium manganate (VII) turns colourless. Bubbles of gas formed. [Both observations must be recorded to get 1M] 9

2 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. Bubbles of gas formed. Gas relights glowing splint. Gas is oxygen. Test-tube feels warm. [Any 2 observations must be recorded to get 1M] 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 10 drops of C with shaking. Colourless acidified KI solution turns brown. [1M] Purple acidified potassium manganate (VII) turns colourless. [1M] Markers’ Comment: This is a standard phrasing to be used for testing of reducing/oxidising agents. Tes