1
EXPERIMENTB7:GREENCRYSTAL
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Synthesizeaninorganicsaltandpurifyitusingthemethodof
recrystallization.
2) Testthepurityofthesynthesizedcrystals.
3) Analyzethesynthesizedcrystalsandpredictitsformulausingmethods
suchas:titration,spectrophotometry,anddeterminationofpercent
hydrate.
Introduction
Oneofthemostexcitingaspectsofchemistryistheabilitytosynthesizenew
materials.Thesesyntheticmaterialsareinavarietyofconsumerproductsandcan
befoundinproductssuchasfood,healthandbeautysupplies,andpharmaceuticals.
Chemistsemployconceptualchemicalmodelstoformulaterelativelysimple
procedurestosynthesizesuchsubstancesreliablyandinhighyield.Once
synthesized,thesubstancesmustbepurifiedandanalyzedtoconfirmthechemical
identity.Thisensuresthequalityandeffectivenessoftheproductinquestion.
Accordingly,synthesis,purification,andanalysisarethreeessentialmethodologies
everychemistmustbefamiliarwith.
InthisexperimentaGreenCrystalwillbesynthesizedfromiron(III)chlorideand
potassiumoxalateandthenpurifiedviarecrystallization,acommontechniquein
syntheticprocedures.Theexactchemicalformulaofthecrystalwillinitiallybe
unknown,thoughthegeneralformulacanbewrittenasKwFex(C2O4)y(H2O)z.
Titrationandspectrophotometrywillbeusedanalyzethepurifiedmaterialsin
ordertodeterminethepreciseformulaofthecrystal.
Part1:SynthesisofGreenCrystal
Aqueoussolutionsofiron(III)chloridehexahydrateandpotassiumoxalate
monohydratewillbereactedatatemperatureofaround90°CtoproducetheGreen
Crystal.Theunbalancedchemicalequationfortheprocessmaybewrittenas
follows:
FeCl3!6H2O(aq)+K2C2O4!H2O(aq)"KCl(aq)+KwFex(C2O4)y!zH2O(s)
Coolingthereactionmixturewillresultintheformationofgreencrystals.The
crystalsobtainedinthisstepareconsideredasthe“crudeproduct”astheyarenot
pure.Thesolidproductwillbeseparatedfromtheaqueoussupernatantby
2
decantingtheliquid,andthenthecrudeproductwillbepurifiedinthenextstepof
theprocess.
Part2:PurificationoftheGreenCrystal
ThepurificationoftheGreenCrystalisdonethroughaprocesscalled
recrystallization.
Inrecrystallization,thecrudeproductisdissolvedinasmallamountofhotsolvent,
completelydissolvingtheentiresample.Atthisstageboththeimpuritiesandthe
desiredproductaredissolved.Thechoiceofsolventissuchthat,thepureproductis
muchlesssolubleinthecoldsolvent,causingthemtosolidify(recrystallize)asthe
solutioncools,whiletheimpuritiesshouldremaindissolved.Theliquid,containing
theimpurities,canthenberemovedviafiltration,leavingthepuresolidproduct
behind.
Inthisexperiment,themostlikelyimpuritiescombinedwiththepureproductare
thestartingmaterials.Accordingtothesolubilityrules,chloridesalts(suchas
FeCl3!6H2O)andpotassiumsalts(suchasK2C2O4!H2O)arehighlysolubleinwater.
Thedesiredproduct,theGreenCrystal(KwFex(C2O4)y!zH2O)isasparinglysoluble
saltthatonlydissolvesinwaterathightemperatures.Therefore,waterisagood
solventchoicefortherecrystallizationprocess.
Thequalityofthecrystalsisimprovedifthecoolingisdoneslowly.Giventhis,itis
notrecommendedthatthecoolingbedoneovericeinitially.Itisbesttoallowthe
solutiontocooltoroomtemperatureslowlyandthenplacethesolutioninanice
bathonlyifnecessary.
Thepercentagepurityoftherecrystallizedproductcanbegreatlyincreasedifthe
processisrepeatedmorethanonce.Whilemultiplerecrystalliztionsincreasethe
qualityoftheproduct,eachiterationalsodecreasesthequantityofproductthatcan
beisolated,resultinginanoveralldiminishedpercentyield.Giventhisconcernonly
oneortworecrystallizationsaretypicallycarriedoutinatypicalsyntheticscheme.
Part3:AnalysisofthePercentofOxalateintheGreenCrystal
Thepercentofoxalate(C2O42-)intheGreenCrystalwillbeanalyzedbytitration(see
experimentsA7andA9forareviewoftitration).Oxalatereactswithpermanganate
inanacidicmediumaccordingtothefollowingequation:
2MnO4−(aq)+5C2O42-(aq)+16H+(aq)"2Mn2+(aq)+10CO2(g)+8H2O(l)
Theabovereactionisanexampleofaredoxtitration.TheMnO4−isreducedtoMn2+
andtheC2O42-isoxidizedtoCO2.Anintroductiontooxidation-reduction(redox)
reactionscanbefoundintheTypesofReactionschapterofanyGeneralChemistry
textbook,whileamoredetailedtreatmentmaybefoundinthelater
3
electrochemistrychapter.Bothofthesesectionswillbediscussedoverthecourseof
theGeneralChemistryseries.
Theprimaryfocusforthepresentisthefactthataccordingtothestoichiometryof
thereaction,2molesofMnO4−willcompletelyreactwith5molesofC2O42-.
Duringthistitration,aknownmassoftheGreenCrystalwillbedissolvedinwarm
acidicwater.Thissolutionwillbecolorlessinitially.Astandardizedpermanganate
solution(whosemolarityispreciselyknown)willbeslowlyaddedfromaburetinto
asolutioninwhichsomeoftheGreenCrystalwasdissolved.Thepurplecolorofthe
permanganatewilldisappearasitreactswiththeoxalateinthereactionflask.Just
pasttheequivalentpoint,whenalltheoxalatehascompletelyreacted,additionof
onemoredropofthepermanganatewillleaveanexcessofpermanganateinthe
solutionandthereforeapalepurplecolorinthereactionflask.Thisistheendpoint
ofthetitration.
Thevolumeofpermanganatesolutionneededforthetitrationandthemolarityof
thepermanganatewillbeusedtodeterminethemolesofpermanganateusedforthe
titration.Fromthestoichiometricrelationshipbetweenthepermanganateandthe
oxalate,themolesoftheoxalatetitratedcanthenbedetermined.Thisisthenused
tocalculatethemasspercentofoxalateinthesample.
Part4:AnalysisofPercentofIroninGreenCrystal
TheanalysisofthepercentageofironintheGreenCrystalwillbedoneusingthe
methodofspectrophotometry.
SpectrophotometryandBeer’slawhavebeenextensivelydiscussedinprevious
experiments(seeExperimentsB4andB6).
TheironintheGreenCrystalhasanoxidationstateof+3.Theanalysisisgreatly
facilitatedbyconvertingalltheFe(III)toFe(II).Inordertoreducetheironfromits
+3to+2oxidationstate,severalstepsarerequired.Thesampleisfirstplacedinan
ammoniumacetatesolution,whichactsasabuffertostabilizethepHofthe
solution.ThereductionofFe(III)toFe(II)isdonebyaddinghydroxylamine
hydrochloride.Finally,thereagentortho-phenanthrolineisaddedtotheFe(II)to
givethesolutionanorangecolor.Ortho-phenanthrolineactsasaLewisbasehere,
donatingelectrondensitytotheironcation,formingtheLewisadduct[Fe(phen)3]2+,
called“ferroin.”Thisorangecoloredcomplexiscommonlyusedforthephotometric
determinationofFe(II)concentrations.
Theorangecoloredferroincomplexhasawavelengthofmaximumabsorbance
(λmax)at510nm.Therefore,spectrophotometricanalysisofthesampleiscarried
outat510nm.
4
InthepreviousdiscussionsofspectrophotometryandBeer’slaw,thecalculationof
theconcentrationofasamplewhoseabsorbanceisknownusestheformula:
A=ε×C×l
Intheaboveformula:Aistheabsorbanceofthesample,Cistheconcentration,lis
thepathlengthoftheinstrumentandεisthemolarextinctioncoefficientofthe
substancebeinganalyzed.
Theaboveformulacanonlybeusedifthemolarextinctioncoefficientofthesample
isknown.Intheabsenceofthisinformation,additionalstepsmustbeemployedto
determinetheconcentrationoftheunknownsample.Oneofthesimplestmethods
involvesconstructingastandardcurveusingknownconcentrationsoftheunknown.
Thisstandardcurveisaplotoftheabsorbanceofknownconcentrationsofthe
analyte,andisoftenreferredtoasa“Beer’sLawPlot.”
Aplotofabsorbance(y-axis)vs.concentration(x-axis)shouldresultinacurve
whoseregressionequationcorrespondstothatofastraightline,y=mx+b.Upon
measuringtheabsorbanceoftheunknownsample,theregressionequation
obtainedfromthestandardcurvewillbeusedtocalculatetheconcentrationofthe
unknown(concentrationx=(y–b)/m).
Ineffect,theslopevalueobtainedinthisgraphshouldequaltheproductofthepath
lengthandthemolarextinctioncoefficient,ε×l.Ifdesired,thevalueofthemolar
extinctioncoefficientcanbedeterminedatthispointaswellbydividingtheslopeby
thepathlength.
Bywayofexampleastohowthiswouldbeemployedinanexperiment,assumethat
astocksolutionoftheanalytewithaconcentrationof1mg/mlhasbeenprovided.
Theknownsolutioncanbedilutedtoobtainasetofsolutionsofknown
concentrationssuchas:0.2,0.4,0.6,0.8,and1.0mg/ml.Theabsorbance
measurementofeachofthesesolutionsandtheunknownsolutionmayresultina
datasetsuchasispresentedbelow:
Solution(mg/ml) Absorbance
0.0
0.012
0.2
0.222
0.4
0.380
0.6
0.612
0.8
0.790
1.0
0.995
Unknown
0.585
5
Inordertoascertaintheconcentrationoftheunknown,astandardcurvemustbe
constructed.Asmentionedbefore,thestandardcurveisaplotofabsorbance(yaxis)vs.concentration(x-axis).Thegraphbelowisthestandardcurveobtained
fromtheabovedata.Theequationofthebest-fitline(seeExperimentB2)isalso
showninthegraph.
StandardCurve
1.2
y=0.9787x+0.0125
R²=0.99857
Absorbance
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
Concentration(mg/ml)
Sincetheabsorbanceoftheunknownsampleis0.585,itsconcentrationiscalculated
usingtheequationofthelineasfollows:
y = 0.9787x + 0.0125
y − 0.0125
0.9787
0.585 − 0.0125
mg
x=
= 0.58 ≈ 0.6
0.9787
ml x=
Part5:AnalysisofPercentHydrateinGreenCrystal
€
FromtheformulaoftheGreenCrystal,KwFex(C2O4)y!zH2O,itshouldbeapparent
thatthesubstanceisahydrate.AswasdiscussedinExperimentA3,hydratesare
inorganiccompoundswithwatermoleculesboundtothem.Inahydrated
compoundafixednumberofwatermolecules,calledthewaterofhydration,are
chemicallycombinedwiththemetalion.
Inthispartoftheanalysis,themasspercentofwaterinthehydratedGreenCrystal
willbedeterminedusingmethodsoriginallydescribedinExperimentA3.The
procedureinvolvesmeasuringthemassofthesubstancebeforeandafteritis
dehydratedbyheatinginordertodeterminethemassofwaterlostintheprocess.
Part6:AnalysisofPercentPotassiuminGreenCrystal
Thedeterminationofmasspercentsofiron,oxalate,andwaterwasdiscussedin
Parts3,4,and5,respectively.Themasspercentofpotassiumisthencalculatedby
difference.Noseparateanalyticalprocedureisneededforthisdetermination.
6
7
ExperimentalDesign
Thisexperimentwillbeperformedoveraperiodoffourlaboratorydays.
Day1:Iron(III)chloridehexahydrateandpotassiumoxalatemonohydratewillbe
combinedinanaqueousmediumatatemperatureof90°Candcooledtoobtainthe
crudeproductoftheGreenCrystal.Thecrudeproductwillthenberecrystallized
usingwaterasthesolventandcooledovernighttoobtainthepureformoftheGreen
Crystal.
Day2:ThemasspercentofoxalateinthepurifiedGreenCrystalwillbedetermined
bytitrationwithstandardizedpotassiumpermanganate.
Day3:ABeer’sLawPlotwillbeconstructedandthemasspercentofironinthe
purifiedGreenCrystalwillbedeterminedbyspectrophotometricanalysis.
Day4:ThemasspercentofwaterinthepurifiedGreenCrystalwillbedetermined
bymeasuringthemassofthehydratedanddehydratedsalt.
ReagentsandSupplies
SolidIron(III)chloridehexahydrate,solidpotassiumoxalatemonohydrate,0.04M
StandardizedKMnO4,3MH2SO4,Standardironsolution(0.05mg/ml),10%
hydroxylaminehydrochloride,3%o-phenanthroline,1Mammoniumacetate
solution
Filterpaper,filterflask,Hirschfunnel,25-mLburet,hotplate,seven10-mL
volumetricflasks,two25-mLvolumetricflasks,Spectrophotometer,cuvettes,micro
crucible
(SeepostedMaterialSafetyDataSheets)
Procedure
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PART1:SYNTHESISOFGREENCRYSTAL
1. Obtainahotplatefromthestockroom.
2. Measure3-4gramsofiron(III)chloridehexahydrateandtransferittoa250-mL
beaker.Recordtheexactmass.
3. Add15mLofdeionizedwatertotheiron(III)chloridehexahydratetodissolve.
Warmslightlyifnecessary.
4. Measure8-9gramsofpotassiumoxalatemonohydrateandtransferittoa250mLbeaker.Recordtheexactmass.
5. Add30mLofdeionizedwatertothepotassiumoxalatemonohydratetodissolve.
Heatthesolutiontoboiling.
6. Transfertheiron(III)chloridehexahydratesolutiontothebeakercontainingthe
boilingpotassiumoxalatemonohydrate.
7. Heatthemixturetoabout90°C.
8. Whenthemixturereaches90°C,fromheatusingtongs.Placethebeakeronared
tileandallowittocoolslowlyforfiveminutes.
9. Prepareatraywithice.
10. Placethebeakercontainingthemixtureofreagentsoniceandallowittocoolfor
30-40minutes.
11. Oncecrystallizationiscomplete,slowlydecantthesupernatantanddiscardthe
liquidintoanappropriatewastecontainerprovidedbytheinstructor.
12. Thesolidcrystalsarethecrudeproduct.Taketheseforwardtothenextstep.
9
PART2:PURIFICATIONOFTHEGREENCRYSTAL
1. Heatapproximately25mLofdeionizedwatertonearboiling.
2. AddacoupleofmLofhotwaterfromStep1tothecrudeproductfromPart1
andheatthismixturealongwiththedeionizedwater.Continueaddingsmall
incrementsofhotwatertothemixturecontainingthecrudeproductuntilthe
crudeproductiscompletelydissolved.Makesurenottoaddtoomuchwater.
[NOTE:Inorderfortherecyrstalizationtobesuccessful,thecrudeproductmust
bedissolvedinaminimumamountofhotsolvent.]
3. Coverthebeakerwithawatchglass.
4. Placethebeakercontainingthedissolvedcrudeproductinsidethelocker.
Purifiedcrystalswillformasthesolutioncools.
5. ThisisagoodstoppingpointforDay1activities.
Day2
6. Coolasquirtbottlecontainingdeionizedwaterinanicebath.
7. ObtainthebeakercontainingthepureGreenCrystalsfromthelocker.
8. SetupavacuumfiltrationusingaHirschfunnelandafilterflask.
9. TransferthecrystalsfromthebeakertothefilterpaperontheHirschfunnel.
Scrapeanycrystalsstucktothebottomofthebeaker.
10. Rinsethebeakerwithtwosmallaliquotsofice-coldwaterandaddthistothe
crystals.
11. RemovethefilterpapercontainingthecrystalsfromtheHirschfunnelandplace
onawatchglass.
12. Allowthecrystalstoairdry.
13. WhilethecrystalsaredryingstartworkingonPart3andreturntoattendtothe
crystalswhentheyarecompletelydry.
14. Obtainthemassofthedrycrystals.Besuretoweighallthecrystalssynthesized.
15. Transferthecrystalsintoalabeledbeakerandplacetheminthelockeruntilthe
endoftheexperiment.
10
PART3:ANALYSISOFTHEPERCENTOXALATEINTHEGREENCRYSTAL
1. Obtainabout20mLofstandardizedKMnO4solution.Recordtheexactmolarity
oftheKMnO4.
2. Obtaina25-mLburetandahotplatefromthestockroom.
3. Setupaburetstandandclamptheburettothestand.Turnonthehotplate.
4. RinsetheburettwicewithdeionizedwaterandconditiontheburetwithKMnO4.
5. FilltheburetwiththeKMnO4solutionandrecordtheinitialburetreading.
6. Measure0.1to0.12gramsofpureGreenCrystals.Recordtheexactmass.
7. Transferthecrystalstoa125-mLErlenmeyerflask.
8. Addabout12.5mLofdeionizedwaterand2.5mLof3Msulfuricacidtothe
Erlenmeyerflask.Addaboilingchipintotheflask.
9. GentlyheattheErlenmeyerflaskonthehotplateuntilthecrystalsdissolve
completely.
10. PlacetheErlenmeyerflaskcontainingthehotsolutionbelowtheburet
containingtheKMnO4solutionandbegintitrating.Constantlyswirltheflaskto
thoroughlymixthecontents.Stopthetitrationattheappearanceofapermanent
palepurplecolor.
11. Recordthefinalburetreading.
12. Repeatsteps6to12anadditionaltwotothreetimes,asneeded.Youshould
have2trialsinwhichthemasspercentagreeswithin10%.
13. Discardallthewasteinanappropriatewastecontainerprovidedbythe
instructor.
11
PART4:ANALYSISOFPERCENTIRONINGREENCRYSTAL
NOTE:Thispartoftheexperimentwillbeperformedingroupsoftwo.The
instructorwillobtainthedataforthestandardcurvethatwillbeusedbytheentire
class.Bothmembersofthegroupmustpreparetheirownsampleforanalysis.
1. Eachgroupshouldobtaintwo10-mLvolumetricflasksandtwo25-mL
volumetricflasksfromthestockroom.
2. Obtaintwocuvettesandaspectrophotometer.Powerupthespectrophotometer
andallowtheinstrumenttowarmupfor10minutes.Theinstructorwill
demonstratetheproperuseofthespectrophotometer.
PreparationofStandards(Steps3-9willbedonebytheinstructor)
3. Labelfive10-mLvolumetricflasksas1,2,3,4,and5andaddthefollowing
reagentsintotheflaskusingelectronicpipetsfordispensingthereagents.
Flask
Iron-standard, Ammonium
Hydroxylamine OµL
acetate,µL
hydrochloride, phenanthroline,
µL
µL
1
100
100
100
100
2
200
100
100
100
3
300
100
100
100
4
400
100
100
100
5
500
100
100
100
4. Fillthevolumetricflaskswithdeionizedwateruntilthecalibrationmark.Iftoo
muchwaterhasbeenadded,discardthissolutioninthewastecontainerand
redothepreparation.
5. Capthevolumetricflask,ensurethatthecapissecure,andthoroughlymixthe
contentsoftheflaskbyinverting.
6. Preparefive“Blank”solutionscorrespondingtoeachstandardsolutionby
substitutingtheIron-standardwithanequivalentamountofdeionizedwater.
7. Measuretheabsorbanceofthesamplesat510nm.
8. Addtheblanksolutiontooneofthecuvettes.Placethecuvettecontainingthe
blankinsidethesamplecompartment,aligntheguidemarkonthecuvettewith
theguidemarkatthefrontofthesamplecompartment,closethelidandadjust
theabsorbancereadingtozero.
12
9. Addthestandardfromvolumetricflask1toacuvette.Nowplacethecuvette
containingthestandardinsidethesamplecompartment,aligntheguidemarkon
thecuvettewiththeguidemarkatthefrontofthesamplecompartment,close
thelidandrecordtheabsorbance.
PreparationofSamples(individually)
10. Measure0.01gramsoftheGreenCrystal.RecordtheexactmassoftheGreen
Crystal.Transferthecrystalstoasmallbeaker.
11. Add10mlofdeionizedwatertothebeaker.
12. Add0.5mlof3Msulfuricacid,usingamicroburet.
13. Heatthebeakertillallthesaltdissolves.
14. Transferthecontentsintoa25-mLvolumetricflask.Rinsethebeakerwith
deionizedwaterandtransfertherinsesolutionintothevolumetricflaskaswell.
15. Fillthevolumetricflaskwithdeionizedwateruntilthecalibrationmark.Iftoo
muchwaterhasbeenadded,preparethesolutioninthisflaskagain.
16. Capthevolumetricflask,ensurethatthecapissecure,andthoroughlymixthe
contentsoftheflaskbyinverting.
17. Carefullytransfer0.50mLoftheabovesolutionintoa10-mLvolumetricflask
labeledas“sample”.Useamicroburetforthemeasurement.
18. Tothe10-mLvolumetricflaskcontainingthesamplealsoadd(useelectronic
pipets):
a. 100µLammoniumacetate
b. 100µLhydroxylaminehydrochloride
c. 100µLO-phenanthroline
19. Fillthevolumetricflaskwithdeionizedwateruntilthecalibrationmark.Iftoo
muchwaterhasbeenadded,preparethesolutioninthisflaskagain.
20. Capthevolumetricflask,ensurethatthecapissecure,andthoroughlymixthe
contentsoftheflaskbyinverting.Thiswillbethe“Sample”.
MeasurementofAbsorbance
21. Setthewavelengthofthespectrophotometerto510nm
13
22. Inalargetesttubepreparea“Blank”solutionbyaddingthefollowing,followed
bythoroughmixing:
a. 100µLammoniumacetate
b. 100µLhydroxylaminehydrochloride
c. 100µLO-phenanthroline
d. 10mLofdeionizedwater
23. Measuretheabsorbanceofthe“Sample”(fromStep20)accordingtomethods
describedinSteps7-9.Usethe“Sample”insteadofthe“Standards”forthe
measurement.
14
PART5:ANALYSISOFPERCENTHYDRATEINGREENCRYSTAL
1. Obtainamicrocruciblefromthestockroom.
2. Heatanemptymicrocrucibleandbringittoaconstantmass(±0.0010g).In
ordertodothis,recordthemassoftheemptycrucible.Thenheatthecrucible
for3-4minutes.Oncethecrucibleiscool,recorditsmassagain.Repeatthe
heating-cooling-weighingprocessuntilthemassrecordediswithin1mgofthe
previousmeasurement.
3. AddsomeGreenCrystaltothemicro-crucible(scoopasmallamountwitha
spatula,approximately0.1000grams).Measurethemassofthemicro-crucible
withthesolid.
4. Heatthemicro-crucibleoveraBunsenburner(useayellowflame).Continuethe
heatingprocessforaboutfiveminutes.Makesurethatthesaltdoesnotturn
blackincolor(whichisanindicationofdegradationoftheproduct).
5. Coolthemicro-crucibletoroomtemperatureandmeasurethemassofthe
micro-cruciblewiththeanhydrouscompound.
6. Inordertoensurethatallthewaterofhydrationhasbeenevaporatedfromthe
solid,themicro-cruciblecontainingtheanhydroussolidshouldbeheatedover
theflameonceagain.
7. Onceagain,coolthemicro-crucibleandmeasurethemassofthemicro-crucible
containingtheanhydrouscompound.
8. Theheating-cooling-weighingofthemicro-cruciblewiththeanhydrous
compoundmustberepeateduntilthemassofthemicro-cruciblecontainingthe
anhydrouscompoundiswithin1mgofapreviousmeasurement.
9. Discardthecontentsofthemicro-crucibleintheappropriatewastedisposal
container.
15
DataTable
PART1:SYNTHESISOFGREENCRYSTAL
MassofIron(III)chloridehexahydrate, grams
Massofpotassiumoxalatemonohydrate, grams
PART2:PURIFICATIONOFTHEGREENCRYSTAL
MassofpureGreenCrystalsynthesized, grams
PART3:ANALYSISOFTHEPERCENTOXALATEINTHEGREENCRYSTAL
MolarityofstandardizedKMnO4
Trial1
Trial2
InitialburetreadingofKMnO4(mL)
FinalburetreadingofKMnO4(mL)
MassofpureGreenCrystals(grams)
Trial3
16
PART4:ANALYSISOFPERCENTIRONINGREENCRYSTAL
MassofGreenCrystal(grams)
Concentrationofstockironsolution
Absorbance
Standard1
Standard2
Standard3
Standard4
Standard5
Sample
Absorbance
PART5:ANALYSISOFPERCENTHYDRATEINGREENCRYSTAL
Constantmassofemptymicro-crucible
(grams)
Massofmicro-crucible+GreenCrystal
(grams)
Massofmicro-crucible+anhydrousGreen
Crystal(followingfirstheating)(grams)
Massofmicro-crucible+GreenCrystal
(followingsecondheating)(grams)
Massofmicro-crucible+GreenCrystal
(followingthirdheating)(grams)
Massofmicro-crucible+GreenCrystal
(followingfourthheating)(grams)
Trial1
Trial2
Trial3
17
DataAnalysis
PART3:ANALYSISOFTHEPERCENTOXALATEINTHEGREENCRYSTAL
MolarityofstandardizedKMnO4
Trial1
Trial2
InitialburetreadingofKMnO4(mL)
FinalburetreadingofKMnO4(mL)
VolumeofKMnO4(ml)
VolumeofKMnO4(L)
MolesofKMnO4(Molarity×Volume)
Molesofoxalate(usestoichiometry)
MolarMassofoxalate(grams/mole)
Massofoxalate(grams)
MassofpureGreenCrystals(grams)
Masspercentofoxalate
Averagemasspercentofoxalate
Trial3
18
PART4:ANALYSISOFPERCENTIRONINGREENCRYSTAL
1. CalculatetheconcentrationofironineachoftheStandardsamples1through5.
Concentrationofstockironsolution=
NOTE:Eachsamplewasdilutedtoafinalvolumeof10.0mL.Forinstance,
Standard1waspreparedbydiluting100µLor0.10mLofstocksolutiontoa
finalvolumeof10.0mL
ConcentrationofIron AverageAbsorbance
Standard1 Standard2 Standard3 Standard4 Standard5 Sample
Unknown
2. PlottheStandardCurve(Plotofabsorbance(y-axis)vs.concentration(x-axis))
andobtaintheequationofthebest-fitline.
Equationofbest-fitline:
19
3. Calculatetheconcentrationofironinthesample.
4. Theabovenumberistheconcentrationoftheironinthe10-mLvolumetricflask
samplepreparedinStep13oftheprocedure.Thissamplewaspreparedby
diluting0.5mLoftheoriginal25.0mLsampletoafinalvolumeof10.0mL.Using
theconcentrationcalculatedinStep3above,calculatetheconcentrationofiron
inthe25.0mLsample.
5. Calculatethegramsofironinthesample.NOTE:Thevolumeofthesampleis
25.0mLandtheconcentrationofironisthenumbercalculatedinStep4above.
6. CalculatethepercentageofironintheGreenCrystal.NOTE:Themassofthe
GreenCrystalisinthedatatable.
Mass of Iron (g)
% Fe =
× 100 Mass of Green Crystal
€
PART5:ANALYSISOFPERCENTHYDRATEINGREENCRYSTAL
Trial1
Massofemptymicro-crucible(grams)
Massofmicro-crucible+GreenCrystal
(grams)
MassofGreenCrystal(grams)
Constantmassofmicro-crucible+
anhydrousGreenCrystal
(followingfinalheating)(grams)
MassofanhydrousGreenCrystal
(grams)
PercentageofwaterinGreenCrystal
(grams)
AveragepercentageofwaterinGreen
Crystal
20
Trial2
Trial3
PART6:ANALYSISOFPERCENTPOTASSIUMINGREENCRYSTAL
Percentoxalate
Percentiron
Percenthydrate
Thereforepercentpotassium
21
PART7:DETERMINETHEEMPIRICALFORMULAOFTHEGREENCRYSTAL
Assumethereare100gramsoftheGreenCrystal:
Grams
MolarMass
Moles
(grams/mol)
Oxalate(C2O42-)
Iron
Hydrate(H2O)
Potassium
EmpiricalFormulaofGreenCrystal:
22
RelativeMoles
€
23
PART8:DETERMINATIONOFPERCENTYIELDOFTHEREACTION
1. ObtainthecorrectformulaoftheGreenCrystalfromtheinstructor.
FORMULA:
2. WriteacompletebalancedchemicalequationforthesynthesisoftheGreen
Crystal.
3. Basedontheexactmassesofthetwostartingmaterialsusedforthesynthesis
(DataTable:Part1),calculatethetheoreticalyieldoftheGreenCrystal.Besure
toconsiderthelimitingreagent.
4. BasedonthemassofthepureGreenCrystalobtained(DataTable:Part2),
calculatethepercentyieldofthereaction.
Experimental Yield
% Yield =
× 100 Theoritical Yield