CGSS 2023 Chemistry Prelim Paper 3 Answers

1 2023 Preliminary Exam/End of Year Practical Exam answers 1 (a) (i) Results table: Records initial burette readings, final burette readings and volume added with correct headings and units in a titration table [1] Trial 1 Trial 2 Final burette reading / cm3 20.90 20.90 Initial burette reading / cm3 0.00 0.00 Volume of S / cm3 20.90 20.90 Best titration results (✓) ✓ ✓ All burette readings for all accurate titres in titration table are recorded to nearest 0.05 cm3 [1] Titration results: Accuracy for average titre of consistent readings • within 0.20 cm3 of supervisor’s average value [max 2] • within 0.30 cm3 of supervisor’s average value [max 1] Student’s average +0.30 cm3 [1] 20.90 +0.20 cm3 [2] 20.80 Supervisor’s average/cm3 [2] 20.60 -0.20 cm3 [2] 20.40 -0.30 cm3 [2] 20.30 Concordance: At least two titre values are within 0.20 cm3 [1] 5 (ii) Calculation of appropriate average volume of S in 2 d.p from closest titre values (titres should be identified either in the table or by a tick, or in the calculation) e.g. Average volume of S = 20.90 + 20.90 2 = 20.90 cm3 1 (b) (i) No. of moles of sodium hydroxide = 20.90 1000 ×0.120 = 0.00251 mol No. of moles of sodium hydroxide present = (a)(ii) 1000 ×0.120 [1] 1 (ii) No. of moles of hydrochloric acid in 250 cm3 of solution R = 0.00251× 250 25 =0.0251 mol No. of moles of hydrochloric acid in 250 cm3 of solution R = 𝑎𝑛𝑠𝑤𝑒𝑟 𝑖𝑛 𝑏(𝑖) × 250 25 [1] 1

2 (iii) No. of moles of hydrochloric acid that reacted with Q = 100 1000 ×0.75-0.0251 = 0.499 mol No. of moles of hydrochloric acid that reacted with Q = 100 1000 × 0.75 − 𝑏(𝑖𝑖) = Calculates initial amount of acid [1] Subtraction of the amount of acid used from the initial amount of acid [1] 2 (iv) Q + 2HCl → QCl2 + H2 Allowed: Mg + 2HCl → QCl2 + H2 1 (v) No. of moles of Q = 0.0499 2 = 0.0250 mol Relative atomic mass of Q = 0.60 0.0250 = 24.0 No. of moles of Q = (𝑖𝑖𝑖) 2 [1] Relative atomic mass of Q = 0.60 (𝑖𝑖𝑖) 2 = … to 1 d.p [1] 2 (c) As some of the mixture splashed out of the conical flask, some unreacted acid is also lost from the reaction mixture in the conical flask. [1] This would result in a lower amount/volume of sodium hydroxide required to neutralise the remaining acid / This would result in a greater calculated amount of hydrochloric acid that reacted with the metal. [1] This leads to a greater calculated amount of Q and subsequently a smaller calculated value of the relative atomic mass of Q. [1] • describe impact of acid spray on the composition of mixture (in particular, the acid that is unreacted) [1] • how lesser acid present in the conical flask affects the results of titration – whether the volume of NaOH used/amount of NaOH calculated or the subsequent calculations involving hydrochloric acid reacted or leftover [1] • effect on the calculated value of the relative atomi