PRSS Sc(Chem) Paper 1/3 AK

PASIR RIS SECONDARY SCHOOL 2025 Preliminary Examination SECONDARY 4 EXPRESS / 5 NORMAL ACADEMIC SCIENCE (CHEMISTRY) 5086 / 5088 MARKING SCHEME MCQ Question 21 22 23 24 25 26 27 28 29 30 Answer C D D D C B C A A B Question 31 32 33 34 35 36 37 38 39 40 Answer C C C D D B B A A B SECTION A 1 ai C 1m aii B 1m aiii E 1m aiv A 1m b number of electrons number of neutrons number of protons 1634𝑆𝑆 16 18 16 818𝑂𝑂 2– 10 10 8 Many students did not manage to get the correct number of electrons for the oxide ion. These students often assume that number of electrons is the same as the number of protons (8) or did a subtraction to end up with 6 instead. 4m 2 a fractional distillation Some students gave incomplete answers citing ‘distillation’ without the word ‘fractional’ or identified incorrectly the method as ‘simple distillation’ instead. 1m b A: to measure the temperature of the vapour Many students wrote “measure the temperature” without specifying what temperature are they measuring. This answer is ambiguous as there are students who gave incorrect ideas such as “measure temperature of solution”. B: to (cool and) condense the vapour into the distillate 1m 1m

This part of the question was attempted. c the alcohols are flammable / may catch fire easily This question was poorly attempted. Many students gave incorrect ideas such as electric heater not being able to reach the boiling points of the alcohols, or that heating with the Bunsen burner may break the glass of the round-bottomed flask. 1m d the boiling point of water is lower than that of butanol OR the boiling point of butanol is higher than that of water OR the water would have boiled off Many students gave vague / incomplete answers e.g. indicating that the boiling points of butanol and pentanol is high but not making reference to the water. The key idea here is that water (in the water bath) would have boiled but the boiling points of butanol and pentanol is still not reached yet, and thus cannot be separated. 1m 3 ai Neutralisation Well done 1m aii H+(aq) + OH–(aq) → H2O(l) correct chemical formula correct state symbols Poorly attempted 1m 1m bi exothermic 30 cm3 Generally well done 1m 1m bii Reaction is already completed / All hydrochloric acid has been reacted when 30 cm3 of aqueous NaOH is added. Not enough HCl to react with NaOH suggesting HCl is the LR [1] Limiting reagent is used up - not specific [0.5] Rej: there is excess NaOH unreacted causing the temp to decrease 1m ci no. of moles of hydrochloric acid = 1.5 × ( 25 1000) = 0.0375 mol Poorly attempted 1m cii no. of moles sodium hydroxide no. of moles of hydrochloric acid = 0.0375 mol Poorly attempted Allow ecf 1m ciii concentration of sodium hydroxide = 0.0375 / ( 30 1000) = 1.25 mol/dm3 Allow ecf 1m

Some students did not use the volume of NaOH