Copy of VS 2024 Sec 4 Chem Prelim Paper 3 Answers

1 Victoria School Secondary 4 Preliminary Examination 2024 Suggested Answer to Chemistry Paper 3 Qn Skill Answer Mark 1(a) PDO MMO Results table: experiment volume of P /cm3 volume of Q /cm3 initial temperature of P /°C highest temperature of mixture /°C temperature rise /°C 1 40.00 10 31.5 34.0 2.5 - 3 2 35.00 15 31.5 35.5 3.5 - 4.5 3 30.00 20 31.5 37.0 5.5 - 6.5 4 25.00 25 31.5 39.0 7 – 8 5 20.00 30 31.5 38.0 5.5 - 6 6 15.00 35 31.0 37.0 4.5 – 5.5 7 10.00 40 31.0 35.0 3 - 4 Results: Temperature readings to be recorded to nearest 0.5 °C Ignore if student wrote °C or ‘+’ in the table Reject if student writes ‘-‘ All temperature rise calculated correctly All temperature readings within range of teacher’s value. All results display the correct trend (increase from 1-4, decrease from 5-7, with 5 no more than 4) 1 1 1 1 1(b) PDO Scale chosen is appropriate and is not accurate (uniform scale chosen to use more than half of each axis); no awkward scale 2 Lines of best fit (equal points on both sides of the graph and any anomalous points ignored) BOD if best fit lines do not cross above highest point *first line must pass through origin. Only 1 anomalous point allowed. Axes (include labels with units) All points plotted correctly (allow error to ½ a square) 1 1 1 1

2 1(c) ACE Correct volume of Q read from graph with units shown (24.5 cm3 ± 0.50). Correct temperature rise read from graph with units shown (7.8 ± 0.10) Working on the graph (Penalize if working is missing) 1 1 1(d)(i) ACE 2NaOH + H2SO4  Na2SO4 + 2H2O 1 1(d)(ii) ACE No of mol of NaOH = 24.5/1000 x 1.23 = 0.03014 mol = 0.0301 mol 2 mol NaOH ≡ 1 mol H 2SO4 No of mol of acid in P = 0.03014/2 mol = 0.01507 mol = 0.0151 mol UNDERLINED VALUES TO BE TAKEN FROM STUDENT’S GRAPH 1 1(d)(iii) ACE Volume of sulfuric acid = 50 – 24.5 = 25.5 cm3 1 1(d)(iv) ACE PDO Concentration of sulfuric acid = 0.01507 / (25.5)/1000 = 0.591 mol/dm3 appropriate units in final answers in ((d)(iii) (cm 3), (d)(iv) (mol/dm3) 1 1 1(e) ACE PDO ΔH = 50 x 1.05 x 4.2 x 7.8 = -1719.9 J Correct sign (-) and appropriate significant figures (1 d.p.) in final answers in (e) 1 1 1(f) ACE I agree with student A but disagree with student B. (must state but no marks) Both a weak acid and a monobasic acid would result in a lower concentration of H+ ions and hence a lower maximum temperature. However, the (final) volume of a weak dibasic acid would be the same as that of the sulfuric acid. 1 1

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4 Qn Skill Answer Mark 2a PDO MMO Results table: record initial mass, mass after 2 minutes and final mass, with correct headings and units in a table initial mass of conical flask and acid / g mass of conical flask and contents after 2 minu