20 22 October 2019 Vinegar Titrationmattson.creighton.edu/GenChemWeb/Lab/8_Vinegar.pdfacetic acid is...

Experiment822October2019VinegarTitration

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Objective:Touseanacid-basetitrationtodeterminetheconcentrationofvinegar.

Todaywewillperformourfirsttitrationofanacid

withabase.

Overview:1.  Titrations,neutralizationreactions,andn=MV2.  Calculatingmolarityandmasspercent3.  Procedure:Whatwearedoingtoday4.  Tricksofthetrade5.  Yourlabreport

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…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


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Thisisthebasicsetup

foralltitrations.

Theacidintheflaskisaddedinthebeginningbeforethetitration.Thepurposeofthetitrationistodeterminehowmanymolesofacid

werepresent.

TheNaOH(aq)goesintheburet.Weuseittoaddexactlyone

equivalentofhydroxidetotheacid.

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1.  Titrations,neutralizationreactions,andn=MV0

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Startwiththeburetcarefully

filledtothe0.00mLmark.

Wecanreadtotwoplacespastthe

decimalwitharealburet–asweseein

theclose-up.

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JustliketheMohrpipet,thenumbersgetbiggergoingdown.

Tricky.Tricky.Don’tmessup.

Close-upofarealburet.

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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



Aswestartthetitration,thesolutionintheflaskcontainsanexcessofacid,evenaswestartaddingbase.The

reactionisinstantaneouswitheverydropadded.


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Whenwe’veaddedexactlyoneequivalent

ofhydroxide,thesolutionwillturnpink.

Andwewillbeallstoichiometrical,

whateverthatmeans.

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Infoforcalculations


Weusetheformulan=MxVforNaOH.WewillbegiventhemolarityofNaOH,MNaOH,andwedeterminethevolumeusedfromtheburet.

Thefirstsignofpinkthatpersistsforafewsecondswithstirringmeanswe’vereachedtheequivalencepointandwearereadyforcalculations.

SupposeMNaOHwas0.09858mol/Landweused14.22mL…

nNaOH=MNaOHVNaOH=0.09858mol/Lx0.01422L=0.001402molNaOH

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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


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


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


InfoforIntroduction

3.Procedure:Whatwearedoingtoday.

Ifthemassislessthan1.99ormorethan2.02,youmessedup.Redothevolumetricpipetmeasurement.

Nextpourthevinegarintotheflask.Rinsethelastdropsintotheflaskwith

deionizedwater

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Todaywearedoingthreetrials–repeatingtheexperimentthreetimesandafterthatwewillaveragetheresults.

Eachtrialstartswithmeasuring2.00mLofvinegarintoataredweighing

boatonamini-balance.

Recordthemassofthevinegarsolution–itshouldbeverycloseto2.00g.


Fromeachtrial,wedeterminethemolesofaceticacidandusethattocalculatethemolarityandthemasspercentoftheofaceticacidinthe

vinegar.

Weneedthreegoodtrials.IfyouaddtoomuchNaOH,itwillbetoopink.Write“Over-titrate”inyourlabnotebookandstartoveron

thattrial.

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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


BecauseTrial2waswonky,wedoanother

trialandifitlookssimilartotheothertwo,weaveragethosethree!

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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.

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Weareshootingforaveryfaintpinkthatpersistsfor15secondswithstirring.ThiswillleadtothefewestmolesofNaOH.Howdoyouthinkthatwillaffectmolarityoftheacid?

Askaboutbonuspointsforveryfaintpersistentpinks.

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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


Wearyoursafetyglassestodayanddressforamess.

Thecoversheetsummarizeseverythingthatyouneedtoincludewithyourreport.

Wedothreeseparatecalculationsformoles,

molarityandmasspercentforeachofthethreetrials.


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Solet’stalkaboutsometitrationtricks!

4.Tricksofthetrade

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Usea3x5notecardasalightscoop…

…andcomparewhatyougetwithyourpartner.

Whatdidyouget?

12.22.You?ThesameGood!

Thisishowtopreventburetreadingerrors.

Thelastdropinavolumetricpipetissupposedtostayin

thepipet.It’sbeencalibratedtoworkthatway.

Oneeasywaytostartexactlyatthemarkistodrawthesolutionuppastthemark,

holdyourfingerfirmlyonthetopandholdthetipofthepipetonthebottomofthebeaker.Rollyourfingerslightlyoffthepipetandsolutionwillslowlydrop.

Sodon’tshake,flickorblowitout

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4.Tricksofthetrade

Wecantouchthepipettothesideofthereceivingvesselinordertodislodgeahangingdrop,however.

Thebest“Trickofthetrade”isthatwecanspeedtitrate!RememberhowthevolumesofNaOHaddedweresosimilarinSlide15?Thatletsus“speedtitrate”andgetoutabitearlier.Rememberhowthefirsttitrationtook14.22mL?Thatmeanstheothertitrationswilltakeaboutthesame–sowecanjetinthefirst12or13mLandthenslow

downfortheperfectpink.

Ifyou“think”maybeyoushouldaddonemoredrop,writedowntheburetvolumebeforeyou

do–justincaseitwasabadidea.

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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…

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Thisisprobablyirrelevantforyourlabreport,butlastnightIdreamedIwashuge.

ChemLabwiththeStickPeopleandBirdwascreatedandproducedbyDr.BruceMattson,CreightonChemistry.Enjoyitandshareitifyouwish.

20 22 October 2019 Vinegar Titrationmattson.creighton.edu/GenChemWeb/Lab/8_Vinegar.pdfacetic acid is...

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