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 evolved extinguishes a lighted Colourless pungent gas turned moist red litmus paper blue.Ammonia gas was formed. 112

4 piece of aluminium foil. Heat the contents carefully.Repeat the test using F.splint.Hydrogen gas was formed.Aluminium foil dissolved to form a colourless solution.Aluminium foil dissolved to form a colourless solution.2(c)(ii)ACED is HClF is HNO3 13P 1.Ona piece of filter paper, draw a starting line in pencil about 2.0 cm from the bottom of the filter paper. 2.Dissolve the food supplement powder in excess water and stir thoroughly.3.Use a toothpick to apply a spot of the dissolved food supplement on the starting line.4.Place the filter paper into a beaker containing ethanol/water as the solvent, with the starting line above the solvent.5.Leave the set-up to stand with the set up covered with a lid. The solvent should move up the filter paper and separate the food supplement into the three amino acids.6.Trace the solvent front at the end of the experiment and analyse the chromatogram.7.Apply a locating agent for the separated components to show.8.Measure the distance travelled by each component and the distance travelled by the solvent.9.Calculate the Rf values of each of the components 10.Compare the values with literature values of all amino acids. The amino acids with the same Rf values as the components are the amino acids found in the food supplement powder.Or run chromatograms of amino acids and compare with the food supplement to identify the components.1m – apparatus (filter paper, beaker, lid, solvent)1m – method (labelling start line, solvent front, level of solvent, location of spots on start line, use of locating agent at the end)1m – measurements 1m – analysis of results/conclusion 4

5