Name_________________________
TeamName______________________
CHM111Lab–RedoxTitration–GradingRubric
Criteria
Pointspossible
Pointsearned
LabPerformance
Printedlabhandoutandrubricwasbroughttolab
Safetyandproperwastedisposalproceduresobserved
Followedprocedurecorrectlywithoutdependingtoomuch
oninstructororlabpartner
Workspaceandglasswarewascleanedup
3
2
3
1
2
2(Standardization)
2(Fepill)
2
1
2
20
PostLabquestions
Datarecordedclearlywithproperunits
Calculationsarecorrect;workisshownindetailwithunits.
Question1(workshownclearly)
Question2.
Question3
Total
Subjecttootheradditionalpenaltiesaspertheinstructor
RedoxTitration
Goal
Todeterminethemassofironinsupplementpillusingredoxtitration.
Introduction
Oxidation-reduction reactions (also known as redox reactions) are reactions that usually involve transfer of
electrons.Todeterminethenumberofelectronstransferred,oxidationstatesareassigned.Oxidationstatesofatoms
arenumbersthathelpchemistskeeptrackofelectronsduringareaction.Eachatominanequationcanbeassignedan
oxidationstateaccordingtocertainrules.Iftheoxidationstateofanatomincreasesasyougofromthereactantstothe
productsinanequation,oxidationhasoccurred(electronshavebeenlost);iftheoxidationstatedecreases,reduction
hasoccurred(electronshavebeengained).
Inbalancingredoxreactions,thereactionisoftenbrokendownintohalf-reactions–thereductionhalf-reaction
andtheoxidationhalf-reaction.Forexample:
(1)
MnO4-+8H++5e-
(2)
Fe2+
Mn2++4H2O Fe3++1e-
Inthereductionhalf-reaction(1),manganesehasundergoneadecreaseinoxidationstatefrom+7to+2.Thus
each manganese atom has gained 5 electrons. In the oxidation half-reaction (2), each iron atom has undergone an
increaseinoxidationstatefrom+2to+3–thatis,eachironatomhaslost1electron.(Spectatorionshavebeenleftout
intheseequations.)
Oxidationmustoccuralongwithreduction,andtheatomsthatgainorloseelectronsarecalledthe"redoxpair".
Inaredoxpair,oneelementwillloseelectronsandtheotherelementwillgainelectrons.Thetotalnumberofelectrons
lostandgainedinthereactionmustbeequal.Thereforewemustmultiplythereductionhalfreactionortheoxidation
halfreaction(orboth)byasmallwholenumbertobalancethereaction.Inthecaseabove,wemultiplytheoxidation
halfreaction(2)by5.Oncetheabovereactionsarebalanced,theoverallredoxreactionbecomes:
(3)
MnO4-+5Fe2++8H+
Mn+2+5Fe3++4H2O
In Part A of the experiment today, you will find the exact concentration of a KMnO4 solution by titrating it
against a solution of iron (II) ammonium sulfate hexahydrate (also known by its former name of ferrous ammonium
sulfate), Fe(NH4)2(SO4)2·6H2O where the number of moles of Fe(NH4)2(SO4)2·6H2O is precisely known. This process of
findingthemolarityofasolutionisknownas“standardizing”thesolution.Usingaburet,youwillslowlyaddtheKMnO4
solutiontotheiron(II)ammoniumsulfatesolution.Reaction(3)willoccur.Permanganateion(MnO4-)hasadeepviolet
colorandactsasitsownindicator.Asthetitrationproceeds,theMnO4- (addedfromtheburet)reactswiththeFe2+in
thesolutionpresentintheflask,convertingtheMnO4- toMn2 .WhenthereisnomoreFe2+toreactwith,thenextdrop
ofMnO4-thatisaddedremainsinthesolutiongivingthesolutionafaintpinkcolor.Thisisknownasthe"endpoint"or
equivalencepointofthetitrationandindicatestheendoftheredoxtitration.
InPartBoftheexperimentyouwilluseyourstandardizedKMnO4solutiontodeterminethemilligramsofironin
aniron(Fe2+)supplementpill.Youwillcrushanddissolveanirontablet,thenperformatitrationsimilartotheonein
partItodeterminetheamountofironinit.
LaboratoryActivity
Equipment buret buretclamp 3x250mlErlenmeyerflasks
Chemicals
Fe(NH4)2(SO4)2·6H2O concentratedH3PO4
Procedure
mortarandpestle
funnel analyticalbalance
ringstand
KMnO4solution
irontablet
3MH2SO4
deionizedwater
A.StandardizingtheKMnO4solution
1. Rinseaburet3timeswithdeionizedwater;besuretoletsomerunthroughthetip.
2. Rinsetheburetcarefullywithasmallportion(2mLorless)ofKMnO4(aq).(Usetheplasticpipetteswithmarkingsto
measure2ml)DisposeoftheusedKMnO4(aq)downthesink.Next,filltheburetwiththeKMnO4(aq)to0.00mL.Be
surethatthetipisfilled.
3. Weightwo~0.3gsamplesofFe(NH4)2(SO4)2·6H2Oonananalyticalbalanceandrecordtheexactmassesonthedata
sheet.Placeeachsampleina250mLErlenmeyerflask(usetwoflasks).Besuretogettheentiresampleintothe
flask.(Youcanusesomedeionizedwatertorinseouttheweighdishintoyourflask.)Dissolveeachsamplein~50
mL of deionized water. Using graduated pipettes (not graduated cylinders), add 3.0 mL of 3 M H2SO4, and 2.0 mL
concentratedH3PO4(85%)toeachflask.TheionsintheacidsformacolorlesscomplexwithFe+3insolutionwhich
simplifiesthedetectionoftheend-point.
4. Recordtheinitialreadingontheburet(readthebottomofthemeniscusateye-level)totwodecimalplaces.Startto
addtheKMnO4solution.Atfirst,thevioletdropsofthepermanganatesolutionwilldisappearquicklyintheflask.
Whenthe disappearance of theviolet colorstarts slowing down, add the solutionslowly. Towards the end of the
titration, the solution should be added one drop at a time. When a faint pink color persists for 30 seconds with
constant swirling, the end-point has been reached. A white piece of paper under the Erlenmeyer flask will aid in
detectingcolorchanges.RecordthelevelofKMnO4solutionintheburettothenearest0.01mL.
5. Repeatthetitrationinstep4withthesecondsampleofFe(NH4)2(SO4)2·6H2O.
6. Using the balanced equation, calculate the moles of Fe(NH4)2(SO4)2·6H2O and the molarity of the KMnO4 solution
using the data from each titration. Your molarities for KMnO4 should agree within 4%. If they don't, do a third
titration.Averageyourmolarities.
B.DeterminingtheMassofIroninaPill
7. Obtainoneironsupplementpillandamortarandpestle.Grindthepillintoafinepowderthentransferallofthe
powdertoanErlenmeyerflask.Usedeionizedwatertorinsethecontentsofthemortarintoyourflask.Add75mL
of deionized water, 5.0 mL of 3 M H2SO4 and 3.0 mL concentrated H3PO4 to the flask. Use the plastic pipettes to
measuretheacids.Pillscontainbindersandotheringredientsthatmaynotdissolve.
8. TitratethesolutioncontainingtheirontabletwiththenowstandardizedKMnO4solution.Theprocedureisthesame
asintheabovetitrations.Again,towardstheendofthetitration,theKMnO4 hastobeaddedonedropatatime.It
maytakealongtimetogetapersistentpinkcolorbecausetheironinthetabletdissolvesintothesolutionslowly.
Grindingyourironpillthoroughlyinthemortarandpestlewillassistthereactiontoproceedasrapidlyaspossible.
9. CalculatethemolesofFe2+presentintheunknownsampleandthemilligramsofironinthetablet.
10. Calculateyourpercenterroraccordingtotheequation:
%Error=⏐mgFeinpill-mgFedeterminedbytitration⏐x100
mgFeinpill
11. DrainanyunusedKMnO4(aq)backintothereagentbottle.Rinseyourburetseveraltimeswithdeionizedwater.Be
suretorinseoutthetip.
Calculations
A. CalculatingtheexactmolarityofKMnO4
Knowing the mass and molar mass of iron (II) ammonium sulfate, you can determine the moles of iron (II)
ammoniumsulfate.ThereisonemoleofFe2+ionspermoleofiron(II)ammoniumsulfate.Usingthebalancedequation
(3),youcandeterminethemolesofMnO4-reactedbasedonthereactionstoichiometry(moleratio)andthemolesof
ironused.ThemolarityofKMnO4canbedeterminedbydividingthemolesofMnO4- bythevolumeofaddedinliters.
Thisprocessoffindingtheexactconcentration(molarity)isknownasstandardizingtheKMnO4solution.
B. CalculatingtheamountofIroninthepill.
KnowingtheAverageMolarityoftheKMnO4(frompartA)andtheexactvolumeofKMnO4added,calculatethe
molesofKMnO4added.Usingthebalancedequation(3),youcandeterminethemolesofFeinthepillbasedonthe
reactionstoichiometry(moleratio)andthemolesofKMnO4added.ThegramsofFeinthepillcanbedeterminedfrom
themolesofFe.
Disposal
• contentsofthereactionflasks–sink.
• unusedKMnO4intheburet–emptyintotheKMnO4(aq)reagentbottle.
RedoxTitration:DataSheet
PartA:StandardizationofKMnO4solution
Name________________________
Run1
Run2
MassofFe(NH4)2(SO4)2·6H2O
MolarMassofFe(NH4)2(SO4)2·6H2O
MolesofFe(NH4)2(SO4)2·6H2O
Initialburetreading
Finalburetreading
VolumeofKMnO4added(inL)
MolesKMnO4added
MolarityofKMnO4solution
AveragemolarityofKMnO4solution
Showallcalculationsforonerun
belowclearlyandcompletelyand
includeappropriateunits:
PartB:Calculatingamountofironinthetablet
Initialburetreading
Finalburetreading
VolumeKMnO4added(inL)
MolesKMnO4added
MolesFeinpill
gramsFeinpill
calculatedmilligramsofFeinpill
(not65mg–useyourdata!)
ReportPage1of2
Showallcalculationsbelowclearlyand
completelyandincludeappropriateunits.
RedoxTitration:PostLabQuestions
Name________________________
1. Calculateyourpercenterrorfortheamountofironinthetablet.Theactualamountofironinthepillis65mg.
2. InPartBoftheexperiment,astudentdidnotwaitbeforerecordingthevolumeofKMnO4addedafterseeinga
purplesolutionandproceededtocalculatetheamountofiron.Whilecalculating,thesolutionbecame
colorless.Howwouldthischangethecalculatedamountofiron?Explainhowitwoulddifferfromthetrue
amountofironinthepill.
3. Giventhefollowingredoxequation:
Cr2O72-+6Fe2++14H+
2Cr3++6Fe3++7H2O
Ifittakes27.35mLof0.125MK2Cr2O7totitrate35.0mLofaFe2+solution,whatisthemolarityofFe2+?(Spectator
ionshavebeenleftoutofthebalancedequation)
ReportPage2of2