sl & hl paper 3 section a experimental work question 7

1© Dr Geoffrey Neuss, InThinking http://www.thinkib.net/chemistry

SL & HL Paper 3 Section A Experimental work Question 7 7. 1.203 g of hydrated oxalic acid crystals, (COOH)2.2H2O, were dissolved in distilled water and the total volume made up to 100 cm3. This solution was used to fill a burette. 10.0 cm3 of sodium hydroxide solution of unknown concentration were added to a conical flask and titrated with the oxalic acid solution. Phenolphthalein was used as the indicator. The titration was repeated two more times to determine the average volume of the oxalic acid solution required to reach the end point. (a) A student calculated the concentration of the oxalic acid solution to be 0.134 mol dm−3. He arrived at this answer by multiplying the mass taken by ten to give the mass in 1.00 dm3 of solution and then dividing by the molar mass of oxalic acid.

(10 x 1.203 ) ÷ ((2 x 12.01) + (4 x 16.00) + (2 x 1.01)) = 0.134 mol dm−3.

Outline the major error in the student’s calculation and determine the correct value for the concentration. [2]

(b) Identify the piece of glassware that should be used to ensure the total volume of 100 cm3 is as accurate as possible when preparing a standard solution. [1]

(c) Before placing the oxalic acid solution in the burette the student washed out the burette with distilled water but then omitted to dry it before adding the oxalic acid solution. Explain how this would effect the final result for the concentration of the sodium hydroxide solution. [1]

(d) The student also washed out the conical flask with distilled and omitted to dry it before adding the 10.0 cm3 of the sodium hydroxide solution and then performing the titration with the oxalic acid solution.. Explain the effect this would have on the final result for the concentration of the sodium hydroxide solution. [1]

(e) Oxalic acid is a diprotic acid. Assuming the sodium hydroxide solution has a very similar concentration to the solution of oxalic that was prepared predict how the electrical conductivities of the two solutions will compare. [1]


SL & HL Paper 3 Section A Experimental work Question 6

6. A student performed an experiment to determine the empirical formula of magnesium oxide. She placed 0.196 g of magnesium in a crucible and heated it strongly with a Bunsen burner lifting the lid occasionally to allow some air in. After the magnesium had combusted it was allowed to cool. The mass of the product was 0.299 g.

(a) Show that the student’s results give Mg5O4 for the empirical formula of magnesium oxide. [1]

The student knew that the correct result is MgO. She wondered whether all the magnesium had burned.

(b) Describe one simple chemical test that she could perform to determine whether the product still contained magnesium metal and state the equation for the reaction that would occur if the test is positive. [2]

(c) Suggest how she could ensure that all the magnesium does burn when she repeats the experiment. [1]

Other than the fact that not all the magnesium may have burned, the student also realised that she had made at least four other assumptions. These were:

1. None of the product escaped when the lid of the crucible was lifted. 2. Other than oxygen, magnesium does not react with any of the other gases in the

air. 3. The magnesium was pure. 4. The magnesium did not react with the crucible.

(d) Suggest how the student could perform the experiment using apparatus available in a school laboratory to overcome the first two of these four assumptions. [2]


SL & HL Paper 3 Section A Experimental work Question 5 5. Hydrogen peroxide decomposes according to the equation: 2H2O2(aq) → O2(g) + 2H2O(l)

The catalytic effect of adding solid manganese dioxide powder to an aqueous solution of hydrogen peroxide was studied using the set-up shown below.

In three separate experiments the volume of oxygen evolved was measured over time when no manganese dioxide was added, when 0.100 g of manganese dioxide was added and when 1.00 g of manganese dioxide was added to the same volume of hydrogen peroxide solution. The temperature was kept constant throughout.

(a) In order to add the manganese dioxide the bung had to be taken out of the conical flask and replaced quickly. This altered the pressure in the apparatus and affected the initial reading on the syringe. Suggest a simple modification to the apparatus which would allow the manganese dioxide to be added to the hydrogen peroxide solution without the bung having to be removed. [1]


(b) Manganese dioxide is a black powder and hydrogen peroxide solution is a colourless liquid. Describe what will be observed inside the flask, before during and after the reaction involving manganese dioxide has taken place. [2]

The quantitative results obtained are shown graphically below.

(c) Determine the rate of reaction after one minute when 1.00 g of manganese dioxide had been used as a catalyst. [1]

(d) Suggest why the rate of the reaction does not increase by a factor of ten when the mass of catalyst added is increased tenfold from 0.100 g to 1.00 g [1]

(e) Assuming that the total volume of oxygen evolved by the time the reaction has gone to completion is 100 cm3 (measured at STP), calculate the amount (in mol) of hydrogen peroxide present initially. [1]


SL & HL Paper 3 Section A Experimental work Question 4 4. A student determined the enthalpy of combustion of ethanol using a spirit lamp to heat up a known mass of water as shown in the diagram below:

A mass of 0.553 g of ethanol was combusted. This was measured by filling the spirit lamp with ethanol, and weighing it before and after the experiment, Other than when the ethanol was being burned a cap was placed on the spirit lamp. The temperature of the 250 g of water increased by 10.4 oC during the experiment.

(a) State why it is necessary to use a cap on the spirit lamp apart from when the ethanol is being combusted. [1]

(b) Show that the enthalpy of combustion of ethanol determined from the experiment is equal to – 906 kJ mol-1 if the specific heat capacity of water is taken as 4.18 kJ kg-1 K-1. [2]

(c) Use Section 13 of the data booklet to determine the experimental error. [1]


In his write-up the student listed the following reasons which contributed to the experimental error.

• There was incomplete combustion as the ethanol was being burned in air, not oxygen.

• There was considerable heat loss from the apparatus during the combustion. • The ethanol may not have been completely pure. • The water used in the beaker was tap water, not distilled water. • All of the 0.553 g of ethanol was assumed to have burned. • Only one result was obtained. The experiment should have been repeated

several times and an average taken.

(d) Suggest two more reasons that contributed to the experimental error. [2]


SL&HLPaper3SectionAExperimentalworkQuestion3

3.Apieceofmagnesiuminasolutionofmagnesiumionswasconnectedviaavoltmeterandaswitchtoapieceofcopperinasolutionofcopperionsasshowninthediagrambelow:

(a)Stateandexplainwhatneedstobeaddedtomakethisset-upfunctionasavoltaiccellsothatwhentheswitchisclosedareadingwillbeobservedonthevoltmeter.[1]

(b)Deducethedirectionofflowofelectronsintheexternalcircuitandstatetheoverallequationforthereactionthattakesplacewhenthecellisoperating.[2]

(c)Describewhatwouldbeobservedwhenapieceofmagnesiumisplacedintoanaqueoussolutionofcopper(II)ions.[1]

(d)Avoltaiccellismadefromazinchalf-cell(consistingofapieceofzincinasolutionofzinc(II)ions)andacopperhalf-cell(consistingofapieceofcopperinasolutionofcopper(II)ions.UsetheActivityseriesgiveninSection25ofthedatabooklettodeducewhethertheelectromotiveforceofthiscellwillbegreaterorsmallerthanthecellshownabovewhenbothareoperating.[1]

(e)Describewhatwillbeobservedwhenapieceofcopperisplacedinasolutionofzinc(II)ions.[1]


SL&HLPaper3SectionAExperimentalworkQuestion2

2.Thefollowingexperimentwasusedtodeterminethenumberofmolesofwaterofcrystallisationinwashingsodacrystals,Na2CO3.xH2O.

5.35gofwashingsodacrystalswereplacedinaconicalflaskandexactly50.0cm3of1.00moldm-3hydrochloricacidsolutionwasadded.Acottonwoolplugwasplacedinthemouthoftheflaskandleftinplaceuntilthereactionwascompletelyfinished.Allthecontentsoftheflaskwerethentransferredtoa100cm3volumetricflaskwithwashingsandthetotalvolumemadeuptothegraduatedmarkwithdistilledwater.A10.0cm3sampleofthisnewsolutionwaspipettedintoanewconicalflaskandtitratedwith0.100moldm-3sodiumhydroxidesolutionusingphenolphthaleinastheindicator.12.60cm3ofthe0.100moldm-3sodiumhydroxidesolutionwasrequiredtoreachtheendpoint.

(a)Statetheequationforthereactionofsodiumcarbonatewithhydrochloricacid.[1]

(b)Explainwhyapieceofcottonwoolwasplacedinthemouthoftheconicalflaskwhilethereactionwastakingplace.[1]

(c)Describehowyoucandeterminepracticallythattheendpointofthetitrationhasbeenreached.[1]

(d)Calculatetheamount(inmol)ofhydrochloricacidremainingafterthereactionwiththewashingsodacrystals.[1]

(e)Calculatetheamount(inmol)ofwashingsodacrystalsinitiallyplacedintheconicalflask.[1]

(f)Determinethenumberofmolesofwaterofcrystallisation(x)inwashingsodacrystals.[1]


SL&HLDataresponsequestion7onCompoundXCompoundXisacrystallinesolidthatoccursnaturallyinsomefruits.AnalysisofpurecompoundXshowsthatitcontains68.84%carbon,4.96%hydrogenand26.20%oxygenbymass.ThemassspectrumofcompoundXshowsamolecularionpeakwithm/z=122.Thelowresolution1HNMRspectrumofCompoundXshowstwosignalswithanintegrationtraceratioof5:1.TheinfraredspectrumofCompoundXshowsabroadstrongpeakbetween2500and3000cm-1andasharppeakat1700cm-1.a.i.ShowthatthemolecularformulaofCompoundXisC7H6O2.[2]ii.StatetheinformationthatcanbededucedaboutthestructureofCompoundXfromits1HNMRspectrum.[2]iii.DeducethemolecularstructureofCompoundX.[1]b.ThevaluefortheaciddissociationconstantofCompoundXis6.31x10-5.UsingHXtorepresentCompoundX,statetheequationforthereactionofCompoundXwithwater.[1]c.ThefollowingtableshowsthesolubilityofCompoundXinwateratdifferenttemperatures:

Temperature/oC Solubility/gdm-30 1.718 2.725 3.440 5.575 21.5100 56.3

SuggesthowanimpuresampleofCompoundXcouldbepurifiedinaschoollaboratory.[1]d.CompoundXreactswithpropan-1-olwhenwarmedinthepresenceofasulfuricacidcatalyst.Theinorganicproductobtainedinthisreactioniswater.i.Statethenameofthetypeofreactionthatistakingplace.[1]ii.Deducethemolecularformulaoftheorganicproduct.[1]


SL&HLDataresponsequestion8oncoalCoalisanabundantfossilfuel.Thedifferentgradesofcoalcanbeclassifiedaccordingtothepercentageofcarbontheycontain.

a.Analysisofasampleofcoalshowsthatitcontains90.5%carbon,4.20%hydrogenand0.800%sulfurbymass.Identifythenameofthegradeofcoalthissamplebelongsto.[1]b.i.Assumingnootherelementsinthesampleofcoalcombust,useSection13ofthedatabooklettodeterminethetheoreticalheatcontentof1.00kgofthisparticularsampleofcoal.[2]ii.Suggestonereasonwhythevalueobtainedinb.i.ishigherthanthevaluegiveninthetableaboveforthisparticulargradeofcoal.[1]c.Thecombustionofcoalisoneofthemaincausesofcarbondioxidepollutionintheatmosphere.In2011itisestimatedthatthetotalamountofcoalcombustedworldwidewas7.695x109tonnes,whichadded1.442x1010tonnesofcarbondioxideintotheatmosphere.Assumingthatthecoalcontainedonaverage87%bymassofcarbonandthatallthecarbonburnedtoformcarbondioxide,calculatethepercentageofcoalthatdidnotreleasecarbondioxideintotheatmospherewhenitwascombustedin2011.[2]


d.”Carboncapture”describesdifferentmethodsinwhichcarbondioxideproducedbycoalcombustionispreventedfromenteringtheatmosphere.Onesuchmethodiscalled“mineralsequestration”andinvolvesreactingthecarbondioxidewithnaturallyoccurringmetalsilicatessuchasforsterite,Mg2SiO4.Theequationforthereactionis:Mg2SiO4(s)+2CO2(g)→2MgCO3(s)+SiO2(s)ExplainwhycarbondioxideisagasatSTPwhereassilicondioxideisasolidwithahighmeltingpoint(1600oC).[2]e.Theproductsofcoalcombustioncanalsocauseaciddeposition.i.Stateoneequationtoshowhowtheproductsfromthecombustionofcoalcanleadtoacidrain.[1]ii.OutlinewhyrainwaterneedstohaveapHbelow5.6beforeitisclassifiedas“acidrain”.[1]


SL&HLDataresponsequestion5onVitaminCTheRoyalSocietyofChemistryhasintroducedaglobalexperimentforschoolstudents1todeterminetheamountofvitaminCinvariousfruits.StudentsfirstdeterminehowmanydropsofagiveniodinesolutionarerequiredtoreactwithaknownamountofvitaminCbycalibratingtheiodinesolutionwithaknownamountofvitaminCusingstarchasanindicator.AlltheVitaminCcontentisthenextractedfromaselectedfruitandthenumberofdropsofthestandardisediodinesolutionrequiredtoreactwiththevitaminCisdetermined.TheamountofvitaminCinmgg-1intheselectedfruitisthencalculated.Therelevantequationforthereactioninaqueoussolutionis

ThemolarmassofvitaminC=176.12gmol−1a.i.Deducethetwohalf-equationsfortheoxidationofvitaminCandthereductionofiodineinaqueoussolution.[2]ii.ExplainwhyvitaminCissolubleinwater.[1]b.Astudentinaparticularschoolobtainedthefollowingcalibrationcurve:

1http://www.rsc.org/learn-chemistry/resource/res00001280/measuring-vitamin-c-in-food-a-


ShefoundthatthevitaminCextractedfrom3.04gofafreshredpepperrequired82dropsoftheiodinesolutiontoreactcompletely.i.DeducetheconcentrationofvitaminCinmgg-1ofthefreshredpepper.[2]ii.Itwasdeterminedthat103dropsoftheiodinesolutionhadatotalvolumeof1.00cm3.Calculatetheconcentrationoftheiodinesolutioninmoldm−3.[2]c.IodinecanbeformedbythereactionbetweeniodideandiodateionsinacidicsolutionI−(aq)+IO3

−(aq)+6H+(aq)→3I2(aq)+3H2O(l)Thisredoxreactionisknownasadisproportionationreactionasiodineissimultaneouslyoxidised(from-1tozero)andreduced(from+5tozero).Iodineisinsolubleinwaterbutitdoesdissolveinasolutionofpotassiumiodideasitformsthecomplextriiodideion,I3−.I2(aq)+I−(aq)→I3−(aq)Discusswhetherthisreactionbetweeniodineandiodideionscanalsobeconsideredtobeadisproportionationreaction.[2]

CalibrationcurveofdropsofiodinesolutionagainstmgofvitaminC.

sl & hl paper 3 section a experimental work question 7

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