Experiment822October2019VinegarTitration
0
10
20
30
40
50
zzzzzzz
Objective:Touseanacid-basetitrationtodeterminetheconcentrationofvinegar.
Todaywewillperformourfirsttitrationofanacid
withabase.
Overview:1. Titrations,neutralizationreactions,andn=MV2. Calculatingmolarityandmasspercent3. Procedure:Whatwearedoingtoday4. Tricksofthetrade5. Yourlabreport
2
…andlearnoneofthemostimportanttechniquesusedbychemists,especiallystudentsinGen.
Chem.
Icandotricks.
HC2H3O2(aq)+NaOH(aq)àH2O(l)+NaC2H3O2(aq)
Thisisthebalancedequation.One
“equivalent”ofacidreactswithone“equivalent”ofbase.Forexample,0.10
molacidreactsstoichiometricallywith
0.10molbase.
Inatitration,whentheacidandbasehavereacted
“stoichiometrically”,thatisone-to-one,we
callthattheequivalencepoint.“Stoichiometric”isafun
word.Iliketodropitintopoliteconversation.
aceticacid+sodiumhydroxideàwater+sodiumacetate
InfoforIntroduction 3
1. Titrations,neutralizationreactions,andn=MV
1. Titrations,neutralizationreactions,andn=MV
0
10
20
30
40
50
Thisisthebasicsetup
foralltitrations.
Theacidintheflaskisaddedinthebeginningbeforethetitration.Thepurposeofthetitrationistodeterminehowmanymolesofacid
werepresent.
TheNaOH(aq)goesintheburet.Weuseittoaddexactlyone
equivalentofhydroxidetotheacid.
4
1. Titrations,neutralizationreactions,andn=MV0
10
20
30
40
50
Startwiththeburetcarefully
filledtothe0.00mLmark.
Wecanreadtotwoplacespastthe
decimalwitharealburet–asweseein
theclose-up.
0
10
20
JustliketheMohrpipet,thenumbersgetbiggergoingdown.
Tricky.Tricky.Don’tmessup.
Close-upofarealburet.
5
HC2H3O2(aq)+NaOH(aq)àH2O(l)+NaC2H3O2(aq)aceticacid+sodiumhydroxideàwater+sodiumacetate
Thesodiumhydroxideisintheburetandwecanaddasmuchasweneeduntilwe’veaddedanamount
equivalenttotheacidpresent.
Afewdropsofphenolphthaleintellsuswhenwe’veaddedenoughhydroxide.
Theaceticacidisintheflask.Inourexperiment,weuse2.00mLaceticacid
andabout40mLdeionizedwater–the
volumeofwaterdoesn’tmattertoomuch.
6Infofor
Introduction
1. Titrations,neutralizationreactions,andn=MV
1. Titrations,neutralizationreactions,andn=MV
Aswestartthetitration,thesolutionintheflaskcontainsanexcessofacid,evenaswestartaddingbase.The
reactionisinstantaneouswitheverydropadded.
HC2H3O2(aq)+NaOH(aq)àH2O(l)+NaC2H3O2(aq)
30
40
50
Whenwe’veaddedexactlyoneequivalent
ofhydroxide,thesolutionwillturnpink.
Andwewillbeallstoichiometrical,
whateverthatmeans.
7
Infoforcalculations
HC2H3O2(aq)+NaOH(aq)àH2O(l)+NaC2H3O2(aq)
Weusetheformulan=MxVforNaOH.WewillbegiventhemolarityofNaOH,MNaOH,andwedeterminethevolumeusedfromtheburet.
Thefirstsignofpinkthatpersistsforafewsecondswithstirringmeanswe’vereachedtheequivalencepointandwearereadyforcalculations.
SupposeMNaOHwas0.09858mol/Landweused14.22mL…
nNaOH=MNaOHVNaOH=0.09858mol/Lx0.01422L=0.001402molNaOH
8
1. Titrations,neutralizationreactions,andn=MV
nNaOH=MNaOHVNaOH
=0.09858mol/Lx0.01422L
=0.001402molNaOH
Torepeat,wedeterminedthenumberofmolesofNaOHfromthen=MxVformulausingthevolumeaddedattheequivalencepoint.We
got0.001402molNaOH
Fromtheequationandthe1:1stoichiometry,weknowwehave
thesamenumberofmolesofaceticacid,0.001402molHC2H3O2.
naceticacid=nNaOH
=0.001402molHC2H3O2
Hereistheformulaformolarity.Weknowboththemolesandthevolume!
Maceticacid=naceticacidVvinegar
2.Calculatingmolarityandmasspercent
Solistenup.Theactualconcentration
ofNaOHwillbewrittenonthemarkerboardinlab.Itisnot
0.09858mol/L 9Infofor
Introduction
Maceticacid=0.001402mol0.00200L
Maceticacid=0.7009mol/LWhatshesaid.It’sthesame
thing…Maceticacid=0.701mol/L
(with3sigfigs) 10
Infoforcalculations
2.Calculatingmolarityandmasspercent
Wecanwritemolarityofacetic
acidbyusingsquareparentheses.
Maceticacidis[HC2H3O2]
Maceticacidis[HC2H3O2]
Maceticacid=naceticacid
Withthisformula,weneedtouse0.00200L(2.00mL)forthevolumeofacidusedinthefirstplace.Sotheaceticacidtestedwas0.7009mol/L.
Vvinegar
Masspercentisatadtrickier.Theformulais… Mass%=100%x
maceticacidmvinegarsol’n
Wecaneasilyfigureoutthemassoftheaceticacidusing
m=nxMM,thatismass=molesxmolarmass.
massaceticacid=naceticacidxMMaceticacid
massaceticacid=0.001402molx60.05g/molmassaceticacid=0.08419gHC2H3O2
11Infofor
Introduction
2.Calculatingmolarityandmasspercent
ThemolarmassofHC2H3O2is60.05g/mol.Ooops!Iwasn’tsupposed
totellyouthat.
Sooo,ifwemeasurethemassofthelittlevinegarsolutionas
wego,we’llbeallset…
Ooooo.Nowwehaveonlythreesignificant
figures!Supposeduringourfirsttrial,wedeterminedthemassofa2.00mLsampleofvinegarhadamassof
2.02g
Mass%=100%x=4.17%0.08419gHC2H3O22.02gvinegar
Mass%=100%xmaceticacidmvinegarsol’n
12Infofor
calculations
2.Calculatingmolarityandmasspercent
InfoforIntroduction
3.Procedure:Whatwearedoingtoday.
Ifthemassislessthan1.99ormorethan2.02,youmessedup.Redothevolumetricpipetmeasurement.
Nextpourthevinegarintotheflask.Rinsethelastdropsintotheflaskwith
deionizedwater
13
Todaywearedoingthreetrials–repeatingtheexperimentthreetimesandafterthatwewillaveragetheresults.
Eachtrialstartswithmeasuring2.00mLofvinegarintoataredweighing
boatonamini-balance.
Recordthemassofthevinegarsolution–itshouldbeverycloseto2.00g.
3.Procedure:Whatwearedoingtoday.
Fromeachtrial,wedeterminethemolesofaceticacidandusethattocalculatethemolarityandthemasspercentoftheofaceticacidinthe
vinegar.
Weneedthreegoodtrials.IfyouaddtoomuchNaOH,itwillbetoopink.Write“Over-titrate”inyourlabnotebookandstartoveron
thattrial.
14
STOPbeforethesolutionlookslikeme!
naceticacid=nNaOH=MNaOHVNaOH
Mass%=100%xmaceticacidmvinegarsol’n
Andherearetheaforementionedequationsyouwillusetodothesecalculations.
TrialVNaOHmsolutionnaceticacid[HC2H3O2]Mass%
114.22mL2.02g0.001402mol0.7009M4.17%
214.90mL2.00g0.001469mol0.7344M4.41%314.28mL2.01g0.001408mol0.7039M4.21%
Average
Foreachtrialwedeterminemolarityandmasspercent.Thenweaveragetheresults
fromthreegoodtrials.
Remember[HC2H3O2]isthesameasMaceticacid.Bothmeanmolarityofaceticacidinunitsofmol/L.
SeehowthevolumesofNaOHforTrial2isnotverysimilartotheothertwo.Theyreallyshouldbeifyouaredoingitright…
Thisishowyouknowifoneofthetrialsisprobablybad. 15Infofor
calculations
3.Procedure:Whatwearedoingtoday.
BecauseTrial2waswonky,wedoanother
trialandifitlookssimilartotheothertwo,weaveragethosethree!
16
3.Procedure:Whatwearedoingtoday.
TrialVNaOHmsolutionnaceticacid[HC2H3O2]Mass%
114.22mL2.02g0.001402mol0.7009M4.17%
214.90mL2.00g0.001469mol0.7344M4.41%314.28mL2.01g0.001408mol0.7039M4.21%
Average 0.7025M4.20%
414.26mL2.00g0.001406mol0.7029M4.22%
Infoforcalculations
Weaveragethethreegood
molaritiesandmasspercents.
Trial2wastoopink,wasn’tit?
Showallofyourcalculationsforeachtrialinyourlab
notebook.Ifyoudecidetothrowoutatrial,justdrawa
singlelinethroughit.
It’simportanttorecognizetrialsthatareprobablybadandshouldberepeated.Ifyouusetoomuch
NaOH,thesolutionwillbetoopink.Inthethreetrialsinthe
previousslide,Trial1wastheleastpinkandTrial2wasthemostpink.Yourthreetrialsshouldcomewithin0.5
mLofeachother.
17
3.Procedure:Whatwearedoingtoday.
Weareshootingforaveryfaintpinkthatpersistsfor15secondswithstirring.ThiswillleadtothefewestmolesofNaOH.Howdoyouthinkthatwillaffectmolarityoftheacid?
Askaboutbonuspointsforveryfaintpersistentpinks.
18
3.Procedure
Forthemostpart,wefollowtheprocedureasdescribedinthelab
manual.But…
…youmayprefertoalwaysstarteachtrialwithafullburet–filledtothe0.00mLmark.
That’scool.Youdon’tneedtomakeatablelikeinthelabmanualbut
youcan.
I’mgoingtobeabatfor
Halloween.
Youwillbeturninginresultson-linetoday.Manyofthenumbersaresmall,suchas0.001402M.Youcan
enterthisnumberjustasshown:0.001402M…
BUTifyouuseexponentialnotation,thereisonlyonecorrectwaytoenterthissortofdatasoExcelcan
recognizeit.UsethisEformatandNOspacesatall:
1.402E-3Therearenospacesexcepttoaddunits.
1.402E-3Misok
PickyPicky1.402x10-3doesn’twork.1.402E-3doesn’twork.1.402x10^-3doesn’twork.Incorrectentriesresultinpointloss. 19
3.Procedure:Whatwearedoingtoday.
Wearyoursafetyglassestodayanddressforamess.
Thecoversheetsummarizeseverythingthatyouneedtoincludewithyourreport.
Wedothreeseparatecalculationsformoles,
molarityandmasspercentforeachofthethreetrials.
3.Procedure:Whatwearedoingtoday.
20
Solet’stalkaboutsometitrationtricks!
4.Tricksofthetrade
21
Usea3x5notecardasalightscoop…
…andcomparewhatyougetwithyourpartner.
Whatdidyouget?
12.22.You?ThesameGood!
Thisishowtopreventburetreadingerrors.
Thelastdropinavolumetricpipetissupposedtostayin
thepipet.It’sbeencalibratedtoworkthatway.
Oneeasywaytostartexactlyatthemarkistodrawthesolutionuppastthemark,
holdyourfingerfirmlyonthetopandholdthetipofthepipetonthebottomofthebeaker.Rollyourfingerslightlyoffthepipetandsolutionwillslowlydrop.
Sodon’tshake,flickorblowitout
22
4.Tricksofthetrade
Wecantouchthepipettothesideofthereceivingvesselinordertodislodgeahangingdrop,however.
Thebest“Trickofthetrade”isthatwecanspeedtitrate!RememberhowthevolumesofNaOHaddedweresosimilarinSlide15?Thatletsus“speedtitrate”andgetoutabitearlier.Rememberhowthefirsttitrationtook14.22mL?Thatmeanstheothertitrationswilltakeaboutthesame–sowecanjetinthefirst12or13mLandthenslow
downfortheperfectpink.
Ifyou“think”maybeyoushouldaddonemoredrop,writedowntheburetvolumebeforeyou
do–justincaseitwasabadidea.
23
4.Tricksofthetrade
① First,thecoverpagewithTAinitials.② Next,thetrimmedcopypagesfrom
yourlabnotebookstapledtogether.③ On-lineresultsdueattheendofclass
today.Latesubmissionsarenotgraded–seethesyllabus.
④ Turninlabreporttodayorbeforethestartofclasstomorrow.Latelabsmaynotbegraded–seethesyllabus.
StickpeopleinspiredbyxkcdcartoonsbyRandallMunroe
(www.xkcd.com)
5.Yourlabreport
Fivelittlepumpkinssitting…
24
Thisisprobablyirrelevantforyourlabreport,butlastnightIdreamedIwashuge.
ChemLabwiththeStickPeopleandBirdwascreatedandproducedbyDr.BruceMattson,CreightonChemistry.Enjoyitandshareitifyouwish.