Experiment7pKaandKaofPhosphoricAcid

1March2018Todayweinves-gate

thesecretlivesofthepolypro&cacids.

IknewabirdnamedPolly...

Wait…whichwayarewegoing?

Objec&ve:TodeterminethepKaandKavaluesforphosphoricacid.

2

Overview:1.  CollectdatadirectlyinExcel2.  Makethethreegraphs3.  Interpolateforequivalencepoints4.  InterpolateforpKavalues5.  Labreportformat

Thisweek’sexperimentisalaboratoryprac-cal:Youwillusetheskillsyoulearnedlastweektoperformtheassignedobjec-vewithminimalintroduc-on.

Youcanusethesamespreadsheetasyou

didlastweek.

Umm…Isthisatrick?

1.Collectyour&tra&ondatadirectlyinExcel

YoucanrecordthevolumeandpHdatapairsdirectlyinExcel-insteadofStep4ofthe

Procedure.

!Usethesameformulasforderiva-vesaslastweek.

BeLeryet,useacopyofthespreadsheetfromlast

week,completewiththeformulas!

3

Ooo!TheyaddedcolorsandformaLed

thenumbers!

2.Makingthethreegraphs.

4

Thedatawillproducea-tra-oncurvelikethisone…

…anda1stderiva-veplotlikethisone.

Thefirstderiva-veplotshowsustheapproximateloca-onoftheequivalencepoints.Inthis

example,thefirstoneis~10mLandthesecondoneat~20mL.The3rdequivalencepoint(~30mL)doesnotshowupbecausetheacidistoodilute.

Byinterpola-on,thefirstequivalencepointis9.977mL.

Andthesecondoneis20.182mL.Isthatweird?

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3.Interpolateforequivalencepoints

Some-mesonlyoneofthesecondderiva-vevalues

isuseable.

Inthepreviousexample,eitherequivalencepointcouldbeused.Inthisexample,thesecond

equivalencepointlooksalotbeLer,sothatoneisthebestonetouse.

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3.Interpolateforequivalencepoints

Thatmeanswehavetwoshotsatit!

Supposethesecondequivalencepointwas19.60mL.Thefirstonewouldbe9.80mLjustbedividing19.60by2.

Wealsoneedtoknowthevolumeshalf-waytothefirstequivalencepoint–whichis4.90mL,andhalfwaybetweenthefirstandsecondequivalencepoint–14.70mL.

19.80mLfrominterpola?onofthedata. 7

3.Interpolateforequivalencepoints

The19.60mLcamefrominterpola-onlike

wedidlastweek.

Informa?onfordataanalysis!

VEqPt-1 VEq-Pt-2 8

3.Interpolateforequivalencepoints

Ifbothequivalencepointscanbedeterminedbyinterpola-onlikewesawinSlide5,thesecondequivalencepointshouldbeexactlytwicethatof

thefirstone,but...

…italmostneverturnsoutquitelikethat.Whatwouldcause

theseerrors?Readingtheburet?

IcanalmostseeSpringBreak!

Veq-pt-1 Veq-pt-21/2xVeq-pt-1

3/2xVeq-pt-1

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4.InterpolateforpKavalues

ThepKa1valuecomesfromthepHathalf-waytothefirstequivalencepoint–whichwedeterminebyanotherinterpola-on.

AndpKa2comesfromthepHathalf-waybetweenthefirstandsecondequivalencepoints–

againdeterminedbyinterpola-on.

ThevolumesusedforpKa1andpKa2areshownwith

thegreencrosses.

InfoforIntroduc?on

H2PO4- HPO4

2- PO4

3-H3PO4

Buffer:H3PO4and

H2PO4-

Buffer:H2PO4

-

andHPO4

2-

Buffer:HPO4

2-

andPO4

3-

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4.InterpolateforpKavalues

FYI,theactualpKa1andpKa2valuesare

2.12and7.21.

Stuffthatisgoodtoknow:Themajorspeciespresentthroughoutthe-tra-onincludethreebuffers.

Iseebuffersaplenty.

1111

4.Procedure:Whatwedotoday

Ilikewater.

Stufftodotoday….

①  Wearyoursafetyglasses.Dressforamess.

②  Take-mewri-nganintroduc-oninyourownwordsbeforelab.

③  Recordobserva-onsanddetailsascarefullyaspossible.

④  EnteryourVolumesandpHvaluesintoExcelasyougo.

⑤  Workinpairs.Doone-tra-onandifatleastone2ndderiva-velooksgoodforinterpola-ngtheequivalencepoint,you’redonecollec-ngdata.

⑥  Showallofyourcalcula-onsinvolvinginterpola-on.

⑦  Thecoversheetsummarizeseverythingthatyouneedtoincludewithyourreport.

⑧  KeeppHinwater!ThepKmetersareinaccuratebelowpH=2andabove11.5.

⑨  WriteameaningfulConclusion.Explainwhyyoudecidedwhichequivalencepointtouse.Insomecases,bothwouldwork.Inothercases,oneequivalencepointisbeLerthantheother.HowdidyouryourKavaluecomparewiththeliteraturevalues?

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S&ckpeopleinspiredbyxkcdcartoonsbyRandallMunroe

(www.xkcd.com)

①  First,thecoverpagewithTAini&als.②  Next,thetrimmedcopypagesfromyourlab

notebookstapledtogether.③  Adachgraphsanddata.Staplealltogether.

Reportswithoutgraphswillnotbegraded.④   Turninlabreportbeforeyouleavetoday.

5.YourLabReport

HappySpringBreak!