VS 2024 O Level Pure Chemistry P1+P2 Answer Scheme

1 Victoria School2024 O Level ChemistrySuggested Answer to Chemistry Paper 3QnSkillAnswerMark1(a)(i)PDOMMOResultsTitration number12Final burette reading / cm323.2023.20Initial burette reading / cm30.000.00Volume of B used / cm323.2023.20Best titration results√√1m: correct headings + units & correct subtraction1m: readings to nearest 0.05 cm31m: readings used are within ±0.20 cm31m: readings are within ±0.20 cm3 of supervisor’s readings 41(a)(ii)ACEAverage volume of B = (23.20 + 23.20) / 2 = 23.20 cm3[answer must be 2 d.p.]11(b)(i)ACEAmount of sodium hydroxide used = 23.20/1000 x 0.150 = 0.00348 molAmount of ethanedioic acid in 25.0 cm3 = 0.00348/2 = 0.00174 mol111(b)(ii)ACEAmount of ethanedioic acid in 1 dm3 = 0.00174/25 x 1000 = 0.0696 mol11(b)(iii)ACEMass of ethanedioic acid in 1 dm3 = 0.0696 x (24+2+64) = 6.26 g11(b)(iv)ACEMass of water in 1 dm3 = 8.90 – 6.26 = 2.64 gAmount of water in 1 dm3 = 2.64 / 18 = 0.147 mol 3 s.f for (b)(ii), (b)(iii) and (b)(iv)1111(b)(v)ACEx = 0.147 / 0.0696 = 2 [must be whole no.]11(c)(i)MMONo effect on volume of B. The number of moles of B remains the same even with deionised water added.11(c)(ii)MMOincrease in the mass (no. of moles) of ethanedioic acid calculated.Or no. of moles/ mass of water decreasesHence, X is still rounded off to the same value.112(a)(i)PDOVolume of D / cm315.0Volume of deionised water added to D / cm35.0Volume of gas collected / cm3411m: 1 d.p. for volume of D and water1m for 15.0 and 5.01m for accuracy for volume of gas to whole no./1 d.p.32(a)(ii)MMOMagnesium is the limiting reactant. Magnesium has completely reacted away at the end of the experiment.12(b)(i)ACE

2 57.5 1m: Scale chosen is appropriate with no awkward scale (uniform scale chosen with graph taking up more than half of each axis)1m: Line of best fit - smooth curve- constant at 80-90s- to pass through origin1m: Axes (include labels with units)1m: Points plotted correctly (to half a small square)11112(b)(ii)ACEAverage rate of gas production = 57.5/25 = 2.30 cm3/s2(b)(iii)ACERate of reaction is highest at the start of the reaction(0-40s), with the steepest gradient.As the reaction proceeds, the rate of reaction slows down as the gradient becomes more gentle.At 80s, the reaction is completed and the rate of reaction is zero as the gradient of the graph becomes zero.112(b)(iv)MMO

3 2(c)(i)MMOtestobservations for Dobservations for FAdd about 1 cm depth of D in a clean test tube.Add about 1 cm depth of dilute nitric acid, followed by a few drops of aqueous barium nitrate.Repeat the test using F.No observable changes No observable changesAdd about 1 cm depth of D in a clean test tube.Add about 1 cm depth of dilute nitric acid, followed by a few drops of aqueous silver nitrate.Repeat the test using F.White ppt was formedNo observable changesAdd about 1 cm depth of D in a clean boiling tube.Add about 2 cm depth of aqueous sodium hydroxide, shake the contents and then add a No observable changes /Effervescence of colourless gas.Gas