ThetitrationofAceticAcidinVinegar
InthislaboratoryexercisewewilldeterminethepercentageAceticAcid(CH3CO2H)inVinegar.
WewilldothisbyTitratingtheAceticAcidpresentwithaStrongBase;SodiumHydroxide(NaOH).The
EndpointoftheTitrationwillbedetectedusingaPhenolphthaleinindicator;anacid-baseindicatorthat
changescolorfromcleartopinkingoingfromitsacidicformtoitsbasicform.
AceticAcid(fr.Latinacetumforvinegar)isthemaincomponentofVinegar.Itisacarbonbasedcompound
withasingleionizableproton,makingitanorganicacidofthelargerclassoforganicacidscalledCarboxylic
Acids;organiccompoundswitha–COOHfunctionalmoiety(partsthatcanbesplit).
WewilldeterminetheAceticAcidcontentofacommerciallypreparedWhiteVinegarsolutionusingatypeof
VolumetricAnalysiscalledTitration.
Asnoted,avolumetricanalysisinvolvesmeasuringthevolumeofasolutionofknownconcentrationthatis
neededtocompletelyreactwithananalyte.TheTitrationisperformedbyslowlyaddingthetitranttothe
analytesolutionviatheburetteuntilanEndpointisreached.TheEndpointisrepresentedbysomedistinct
physicalchangeintheanalytesolution;typicallyanIndicatorcolorchange.Iftheindicatorischosenwell,the
EndpointwillrepresenttheEquivalencePointoftheTitrationReaction;thepointatwhichtheaddedamount
oftitrantisstoichiometricallyequivalenttotheamountofanalyte.Byknowingtheconcentrationandvolume
ofthetitratntused,thenumberofmolestitrantcanbedetermined.Thereactionstoichiometrythenallows
ustodeterminetheamountofanalytepresent.
Example:
Inourcase,theAnalyteistheAceticAcidintheVinegarandtheTitrantisadilutesolutionofthestrongbase
SodiumHydroxide.TheEndpointofthetitrationwillbedetectedbyobservingthecolorchangeforvinegar.
LABPREP:
•
Thefollowingstephastobeasaccurateaspossiblesinceitisyourstandardreaction!
UseavolumetricflasktopreparetheNaOHstandardsolutionforaccuracy.Addsomedistilledwatertothe
volumetric flask. Mass out the NaOH in a plastic weighing dish. Carefully rinse the chemical into the flask
withdistilledwaterinarinsebottletogetallthechemicalintotheflask.Swirltheflaskuntilthechemicalis
completely dissolved, then add distilled water until the bottom of the meniscus is exactly on the etched
mark. Stopper the flask and invert at least 25 times to mix. Be sure you make enough solution to do the
completelab.
Topreparea0.600Msolutionofsodiumhydroxide:
For1000mL(1liter)use24.00gramsNaOH
For250mLuse6.00gramsNaOH
For100mLuse2.40gramsNaOH
Procedure:
1.MakeyourownstandardizedNaOHbasefor1stsetoftrialsandrepeatthefollowingproceduresusingthe
StandardizedNaOHsolutionfor2ndsetoftrials.
NOTEONBURETTECHNIQUE:Whenfillingaburet,checktomakesurethestopcockisclosed.Holdtheburet
inyourhandwithapapertowelwrappedaroundittocatchanyspill,andfillslowly.Wipetheburetuptothe
topwiththepapertowel.
2.Pourdirectlyfromthebeakerintotheburetabout5mLoftheNaOHsolution.Rinsethewallsoftheburet
thoroughlywiththeNaOH,allowittodrainthroughthestopcock,anddiscardit.Rinsetheburettwomore
timesinasimilarfashion,usinganew5mLportionoftheNaOHsolutioneachtime.Discardallrinsesolutions.
FilltheburetwiththestandardNaOHsolutionabovethezeromark.andplacetheburetinonesideofthe
doubleburetclamp.Withdrawenoughsolutionfromtheburettoremovetheairfromthejettipandbringthe
liquidlevelintothegraduatedregionoftheburet.Yourstartingvolumedoesnothavetobezero.Discard
whatyouwithdrew.Labeltheburet.
3.Followsteps1and2forthevinegar,andplaceitontheothersideofthedoubleburetclamp.Besureto
labeltheburet.
Titrations:
Makeallburetreadingsto0.01mL
4.ReadtheburetwiththevinegarastheinitialvolumeandrecorditintheDATATABLEfortrial1,asthe
initialvolumeforvinegar.Obtainapproximately10mLofvinegarintheflask.Readtheburetandrecorditin
theDATATABLEfortrial1,asthefinalvolumeforvinegar.
5.Add10or15mLofdistilledwatertotheflasktoincreasethevolumeandmakereadingtheequivalence
pointeasiertoread.Add1or2dropsofphenolphthaleinsolutiontotheflasktoserveasanindicator.
6.ReadtheinitialvolumeoftheNaOHsolutionintheburetandrecorditintheDATATABLEfortrial1,asthe
initialvolumeforNaOH.
7.Placetheflaskwiththevinegarsample,onasheetofwhitepaper,undertheburet.Nowbeginthetitration
byaddingthehydroxidesolutionslowly,whileswirlingtheflask.Apinkcolorwillappearinthecenter,butwill
goawayasyouswirl.Whenthepinkcolorbeginstolinger,addtheNaOHonedropatatime,swirlingbetween
eachdrop.Whenonedropisadded,andthefaintpinkcolordoesnotdisappear,youhavereachedthe
equivalencepoint(thepointwherealltheacidisjustneutralizedbythebase,butnoextrabaseisadded).
Readthevolumeintheburet,andrecorditintheDATATABLEfortrial1,asthefinalvolumeforNaOH.
8.Discardtheliquidintheflaskandrinsetheflaskwith4or5rinsesofdistilledwatertobesureitisclean.
Repeatsteps4-7fortrials2and3.
RecordvolumesfortheburetsintheDATATABLEintheappropriateplaces.
NOTE:Youdonothavetorefilltheburet,ifyouaresureitwillnotgopastthe50mLmarkinatitration,just
readtheinitialandfinalvolumes.Ifyoudogobelowthegraduations,youwillhavetore-dothattitration,as
youwillnotbeabletoreadafinalvolume.
DataTable:
1stsetDATATABLEwithstudentmadeNaOHsolution(0.6M)
BuretReadings(mL)
Vinegar
Trial
NaOH
initial
final
initial
final
1
2
3
2ndsetDATATABLEwithStandardizedNaOH(0.6M)
BuretReadings(mL)
Vinegar
Trial
NaOH
initial
final
initial
final
1
2
3
Calculations:
Set1CALCULATIONSTABLE
Volume
vinegar(mL)
Trial
Volume
NaOH(mL)
Molesof
aceticacid
invinegar(mole)
Moles
NaOH(mole)
1
2
3
Volume
vinegar(mL)
Trial
Set2CALCULATIONSTABLE
Volume
NaOH(mL)
Molesof
aceticacid
invinegar(mole)
Moles
NaOH(mole)
1
2
3
1.CalculatethevolumesofvinegarandNaOHusedforeachofthethreetrials.RecordintheCALCULATIONS
TABLE.
Thisissimply:initialvolume-finalvolume
2.RecordthemolarityoftheNaOHsolutionusedinthetitration.
MolarityofNaOHsolution=_______M
3.DeterminethemolesofNaOHusedineachofthethreetrials.RecordintheCALCULATIONSTABLE.
Bydefinitionmolarity=
molessolute
1Lsolution
DON'TFORGET:YouhavetoconvertthemLofsolutionusedintolitersfortheformula.
4.Thebalancedequationofthereactionbetweenaceticacid(vinegar)andsodiumhydroxideis:
HC2H3O2+ NaOH ---> H2O +NaC2H3O2
1mole 1mole
1mole 1mole
Fromtheaboverelationship,youcancalculatethemolesofaceticacidineachtitration.Recordin
theCALCULATIONSTABLE.
1moleaceticacid
molesNaOHX
=molesaceticacid
1moleNaOH
Becausetherelationshipofsodiumhydroxideandaceticacidinthisreactionis1to1,themolesofaceticacid
willbethesameasthemolesofsodiumhydroxide.
5.Giventhemolesofaceticacidandthevolumesofthevinegarsampleforeachtrial,calculatethemolarities
forthethreetrials.Recordinthespacesprovided.
Don'tforgettoconvertthevolumetoliters.
molesofacid
=molarityofvinegar
volumeofvinegar
Molarityofvinegarfortrial1=_______M
Molarityofvinegarfortrial2=_______M
Molarityofvinegarfortrial3=_______M
14.Calculatetheaveragemolarityofthevinegarsamples.Recordinthespaceprovided.
molaritytrial1+molaritytrial2+molaritytrial3
=Averagemolarity
3
Averagemolarityofvinegar=_______M
6.ThemassofonemoleofHC2H3O2(aceticacid)is60.0g.Rememberingthatmolarityisequaltomoles/liter,
calculatethegramsofaceticacidthatwouldbeinoneliterofyourvinegarsamplebythefollowingformula:
molesHC2H3O2 60.0gHC2H3O2
X
=g/literofHC2H3O2
1Lvinegar
1moleHC2H3O2
MassofHC2H3O2/1Lvinegar=_______g
7.Ifweassumethedensityofvinegarisverycloseto1.00g/mL,thenthemassof1.00Lofvinegaris1000g.
Thepercent,bymass,ofaceticacidcanbeexpressedas:
massofHC2H3O2/1L
X100%=%HC2H3O2(bymass)
1000gvinegar
calculatethepercentageofaceticacidinyourvinegarsample.Record.
Set1: %HC2H3O2=_______%
Set2:%HC2H3O2=_______%