Experiment 5 ·∙ Freezing-‐point depression Pre-‐lab questions Answer these questions and hand them to the TF before beginning work (1) What is the
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[PDF] Molar Mass by Freezing Point Depression
Obtain from your lab instructor a sample of unknown molar mass Record the sample number on your report sheet SAFETY: 1) CAUTION: The unknown sample is
[PDF] Molar Mass by Freezing Point Depression
In this experiment, you will determine the freezing point of cyclohexane and the Kf is called the molal freezing point depression constant and After you have obtained the freezing point data, remove the assembled test tubes from The molal freezing point depression constant for water is 1 86 ˚C kg/mole Setup: Answer
[PDF] Experiment 5 ·∙ Freezing-‐point depression - BU Personal Websites
Experiment 5 ·∙ Freezing-‐point depression Pre-‐lab questions Answer these questions and hand them to the TF before beginning work (1) What is the
[PDF] FREEZING POINT DEPRESSION LAB - District 196
FREEZING POINT DEPRESSION LAB When the freezing point is reached, the temperature will become constant and Post-Lab Questions: (Show all work) Divide your answer to #4 by the atomic mass of sulfur to determine the chemical
[PDF] Freezing Point Depression Lab Answers - scheduleitio
Given the data in the table, answer the questions below and determine the "real" van 't Hoff factor of the solute Lab 3 PostLab - Freezing Point Depression pdf
[PDF] “Molar Mass Determination by Freezing Point Depression” Lab
Watch the lab video for the “Freezing Point” lab, found here: including vapor pressure depression, freezing point depression, and boiling point elevation CALCULATIONS: Determining the Molar Mass of the Benzoic Acid In adding your unknown solute to your solvent, you spilled some solute onto the table after
[PDF] Freezing point depression of acetic acid - Bellevue College
In the following experiment, we will explore the freezing point depression of solutions In Part 1, we acid (HAc) and the equivalent curve after an unknown solute has been added to the solvent Calculations for Part 1 The density of acetic
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Chem 162 Experiment 6 Molar Mass Determination Using Freezing Point Depression Pre-Lab Questions 1 Define the term colligative property 2 Suppose the
[PDF] Purpose: The freezing point depression of a solution is measured to
Experiment 4: FREEZING POINT DEPRESSION 41 Purpose: An molar mass, which is determined by freezing point depression Introduction: When a By measuring the freezing points of the solvent, before and after the unknown is added, the Complete the table and calculations on page 50 Prepare this table in your
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Experiment5·Freezing-pointdepression5-2Experiment5Freezing-pointdepressionBackgroundAphasediagramsuchasFigure5-1indicatesthemoststablestateofasubstance(solid,liquid,orvapor)atagiventempera-tureandpressure.Phaseboundariesonaphasediagramindi-cateconditionsoftemperatureandpressureatwhichtwoormorephasesc oexistinequilibriu m.Whentemperature andpressurechange,asubstancemayundergoaphasetransition,thatis,aconversionfromonestatetoanother;thenamesofthesephasetransitionsappearinFigure5-1.Althoughweareconcernedwiththeconversionofaliquidtoasolid(i.e.,freez-ing)inthisexperiment,wemustfirstdiscusstheconversionofaliquidtoavapor(i.e.,boiling)becauseanunderstandingoftheboilingprocessprovidessometheoreticalideasneededinthepresentexperiment.Mostliquids,ifleftinabeakerthatisopentotheatmos-phere,willintimecompletelyevaporate.However,ifabeakercontainingsufficientliquidisplacedinsideaclosedbox,some,butnotall,oftheliquidevaporatesandtheatmospherewithintheboxbecomespermeatedwithvaporcomingofftheliquid.Becauseitisagas,thevaporexertsapressureontheinte-riorwallsofthebox;thispressureiscalledthevaporpressureoftheliquid.Liquidsthathaveahighvaporpressureatroomtemperaturearesaidtobevolat ile,thatis ,theyevapora te
Experiment5·Freezing-pointdepression5-3readily,whereasliquidst hathavealow vaporpressureat roomtemperaturearesaidtobenon-volatile.Ether,whoseva-porpressureis0.703atmat25°C,isanexampleofavolatileliquid.Water(vaporpressure=0.0313atmat25°C)isconsid-eredamoderatelyvolatileliquid.Mercury(vaporpressure=2.4×10-6atmat25°C)isessentiallyanon-volatileliquid.Boilingoccurswhenthevaporpressureofaliquidequalsthepressureofthesurroundingatmosphere.Ifthepressureofthesurround ingatmosphereis1atm,thetemperatu reat whichtheliquidboilsiscalleditsnormalboilingpoint.Inthe18 80stheFre nchchemistFrançoi s-MarieRaoultdiscoveredthatdissolvingasubstance(thesolute)inaliquid(thesolvent)lowersthevaporpressureoftheliquidsolvent.Raoult'sobservationcanperhapsbebestappreciatedbylook-ingatFigure5-2inwhichwehaveplottedthephasediagramofapuresubstance(solidline)andthephasediagramofaso-lutioninwhichthatpuresubstanceisthesolvent(dottedline).InaccordwithRaoult'sobservation,theliquid-vaporphaseboundarybelongingtothesolutionislowerateverypointthanFigure5-1Aphasediagramshowsthemoststablestateofasubstanceatanygiventempera-tureandpressure.Phaseboundaries(solidlines)indicateconditionsatwhichtwoormorephasescoexistinequilibrium.Tfdenotesthenormalfreezing(ormelting)point,Tbdenotesthenormalboilingpoint,andthetriplepointdenotesconditionsatwhichsolid,liquidandvaporallcoexist.
Experiment5·Freezing-pointdepression5-4theliquid-vaporphaseboundarybelongingtothepureliquidsolvent.Aconsequenceofthisbehavioristhephenomenonofboiling-pointelevation:thesolutionboilsatahighertempera-turethanthepureliquidsolvent.Therequirementthatthevaporpressureofthesolutionbelowerthanthatofthepureliquidsolventimpliesthatthesolu-tionhasadifferenttriplepointfromthatofthepuresubstance:thetriplepointofthesolutionisfartherdownthesolid-vaporphaseboundary.Becaus ethesolid-liquidphaseboundaryoriginatesatthetriplepoint,Figure5-2indicatesthatvapor-pressureloweringalsoresultsinthephenomenonoffreezing-pointdepression:thesolutionfreezesatalowertemperaturethanthepureliquidsolvent.Theadditionofantifreezetothewaterintheradiatorofacarisperhapsthemostfamiliarprac-ticalapplicationoffreezing-pointdepression(seethepost-labquestions).Thefreezing-pointdepressionlawandthevan'tHoffifactorTheincrementbywhichthefreezingpointofasolventisFigure5-2Phasediagramofapureliquid(solidline)andofasolution(dottedline)inwhichtheliquidisthesolvent.Dissolvingasubstance(thesolute)intheliquidlowersthevaporpressureoftheliquidsolvent.Thiseffectresultsinboiling-pointelevationandfreezing-pointdepression.!"#$"%&'(%"
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Experiment5·Freezing-pointdepression5-5loweredbytheadditionofaso luteispropor tional totheamountofsolutepresentinthesolvent.InequationformΔTf=Tf,solution-Tf,solvent=-iKfmwhereΔTfistheamountbywhichthefreezingpointofthesol-ventislowered inunits ofdegreesCelsius,Tf,solutionisthefreezingp ointofthesolution inunit sofdegreesCelsius, Tf,solventisthefreezingpointofthepuresolventinunitsofde-greesCelsius,iisaunitlessnumbercalledthevan'tHoffifac-tor,Kfisthefreezing-pointdepressionconstantofthesolventinunitsofdegreesCelsius·kilogramofsolventpermoleofsol-ute,andmisthemolalityofthesolutioninunitsofmolesofsoluteperkilogramofsolvent.NotethatΔTfisanegativenum-berbecauseTf,solution Experiment5·Freezing-pointdepression5-6Sucrosemoleculesneitherhookupwitheachothernordotheydecomposewhentheydissolv einwater:the yjustgetsur-roundedbywatermoleculesandgointosolution.Thesituationistotallydifferentwhenasaltsuchassodiumchloride(NaCl(s))dissolvesinwater.InthiscaseNaCl(s)dis-sociatesintosodiumions( Na+(aq))andchloride ions(Cl-(aq)): Herethevan'tHoffifactorissetequaltotwo(i.e.,i=2)be-causeonemoleofNaClbreaksapartintoonemoleofNa+ionsandonemoleofCl-ions.Thefollowingexamplesillustratestheroleofthevan'tHoffifactorincalculatingfreezingpointsusingthefreezing-pointdepressionlaw.Example5.1Supposethat4. 00gofsodiumc hlori de(NaCl, MW=58.44g/mol)isdissolvedin473mLofwater(MW=18.02g/mol,d=1.00g/mL).Calculatethefreezingpointofthesolution.AnswerThemolalitymofNaClinthesolutionis Substitutingintothefreezing-pointdepressionlaw(Eqn.5.1)andrecogni zingthatNaCldissociatesinto Na+ionsandCl-ionswhenitdissolvesinwater(i.e.,i=2)gives Experiment5·Freezing-pointdepression5-7Aslongasasaltsolutionisdilute,thecalculatedfreezingpointmatchesthe experimentallydetermin edfreezing pointwell.But,asasaltsolutionbecomesmoreconcentrated,thefreezing-pointdepressionlawappearstomissbadly,asillus-tratedinthefollowingexample.Example5.2When1.05gofthesaltcoppersulfate(CuSO4)isdissolvedin15 .00mLofwaterto makea solutionthatis0.44minCuSO4andthesolution'sfreezingpointismeasured,wefindthatTf,solution=-0.82°C.Calculatethefreezingpointofthesolutionusingthefreezing-pointdepressionlaw(Eqn.5.1)andassumingthatCuSO4dissociatesaccordingtotheequation ThisvalueofiimpliesthatCuSO4,muchlikethetablesugarintheexampleworkeda bove,ismerelyenteri ngsolut ionasCuSO4(aq).Itseemstha ttheion -ionforceof att ract ionbe-tweenCu2+ionsandSO42-ionsisstrongerthantheion-dipoleforceofattractionthatCu2+ionsandSO42-ionshaveforthewatermoleculesthatsurroundtheminsolution:CuSO4preferstosticktogetherratherthanbreakapart.ThatCuSO4doesn'tdissociateintoCu2+ionsandSO42-ionsinwaterissupposedtocomeasashocktoyoubecauseitviolatesourchemicalin-tuitionandeverythingthatwe'vebeentoldsincehighschool!Theimportanceofthevan'tHoffifactoristhatitprovidesinformationaboutwhatisgoingoninasolution.InthecaseoftheCuSO4solutionmentionedabove,avan'tHoffifactorofi=2meansthatthespeciesinsolutiondonotinteractwitheachother;avan'tHoffifactorofi<2meansthatthespeciesinso-lutionareattractedtoeachother;avan'tHoffifactorofi>2meansthatthespeciesinsolutionarerepelledbyeachother.Inthisexperimentyoumeasurevan'tHoffifactorsofvari-oussolutionsinordertomakedeductionsaboutthenatureofthosesolutions. Experiment5·Freezing-pointdepression5-9ProcedureDeterminationofthefreezingpointofadilutesaltsolutionObtainorassembleanapparatusformeasuringfreezing-pointdepression(seeFigure5-3).TheapparatusconsistsofalargePyrextesttubesealedoffwithaneoprenestopperthathasbeenfittedwithathermometer.Clamptheapparatustoaringstand.Usingagraduatedcylinder,measureoutabout15mLofdeionizedwater.Recordthee xactvolumeyoutake inyournotebook.Pour thewaterintothebigtesttube.Weigh outabout1gofcobalt(II)chloride(CoCl2(s),MW=129.83g/mol);recordthemasstotwodecimalsinyournotebook.DissolvetheCoCl2(s)inthewater;CoCl2(s)releasesheatwhenitdissolvesFigure5-3Apparatusformeasuringfreezing-pointdepression. Experiment5·Freezing-pointdepression5-10sodon'tbesurprisedifthetesttubegetsquitehot.Lettheso-lutioncooldowntoaboutroomtemperatureandthensealthetubewiththethermometer-equippedstopper.Youcanaccel-eratethecool-downofthesolutionbyimmersingthebigtesttubeinabeakerofroom-temperaturewater.Youwillnowprepareacoldbath.Obtainasecondther-mometeranda600-mLbeaker;placeabout300mLoficeinthebeaker.Usingacleananddry250mLbeaker,measureoutabout50mLofNaCl(s).AddtheNaCl(s)totheice.Stirthemix-turevigorouslyusingaspatula.Afteraboutaminuteofstir-ring,themixtureshouldgetverycold.Youcancheckitstem-peratureusingthesecondthermometer.Addanother300-mLportionoficeandanother50-mLportionofNaCl(s);stirthemixturevigorously.Youwantthetemperatureofthemixturetostaybelow-15°C.Toinsula tethecoldbathagainstt emperaturec hanges,wrapthe600-mLbeakerinagenerouslayerofpapertowelsandenclosethepaper-towelblanketinaluminumfoil;usetapetokeepthiscrudebuteffectiveinsulationinplace.Youarenowreadytocarryoutyourfirstfreezing-pointde-termination.Beforeyoubegin,besurethatthetipofthether-mometerisdippingstraightdownintothemiddleoftheCoCl2solutioninthebigtesttubeandthatthetipisnotpointingtothesideortouchingthebottom.PlacethetesttubecontainingtheCoCl2solutionintothecoldbath.Immediatelyrecordthetemperatureofthesolutionuponimmersion(callthist=0 sec)andevery20 secthereafterfor10min.Youshouldobservethreeseparaterégimesofcoolingbe-havior:(1)immediatelyafteritsimmersioninthecoldbath,thetemperatureofthesolutionrapidlydecreases;(2)asthesolutionbeginstofreeze,therateatwhichthetemperaturede-creasesshouldslowdownsignificantlyandmayalmostleveloffintowhatwewillcallaquasi-plateau;(3)afterthesolutioniscompletelyfrozen,thetemperaturedecreaseresumesatafasterrate.Itwouldbeanexcellentideatoplotthedata-byhandinyournotebookoronacomputer-asyoucollectitsothatthe threecoolingrég imesareeasiertosee; yourplotshouldresembleFigure 5-4. Experiment5·Freezing-pointdepression5-11Thefreezing pointofthesolutionis thetemperatureat whichthesolutionjustbeginstofreeze.ThequantityTf,solutionisbestfoundbydrawingalinethroughthedatapointscorre-spondingtotheinitialrapidcoolingstageanddrawingasec-ondlinethroughthepointscollectedduringthequasi-plateaucoolingstage:Tf,solutionisthetemperatureatwhichthetwolinesintersect(seeFigure5-4).Ideally,youshould continuetakingtime-versus-tem-peraturedatauntilyouhavepositivelyidentifiedthethirdré-gimeoftemperaturedecrease,butthisisnotalwayseasytodobecausetherecanbesignificantwobbleinthedataandyourcoldbathmaynotbecoldenough.Checkthetemperatureofthecoldbat hbetweentimed measurements.T hecoldbathmustbebelow-15 °C;ifitisnot,stirthesalt-icemixturewithaspatulaalthoughthisoperationwillbeawkwardwiththebigtesttubeintheway.Whateverhappens,continuetotaketime-versus-temperaturedataforatleast10min.Thereliabil ityofyourtime-versus-temperaturedatamayFigure5-4Thecoolingcurveofa0.55mCoCl2(aq)solutionundergoingfreezingexhibitsthreerégimesofdecreasingtemperature:(1)aninitialrapidrateofcoolingfollowedby(2)aquasi-plateauoverwhichtherateofcoolingdecreasesandmayalmostleveloffthen(3)asecondmorerapidrateofcooling.ThefreezingpointofthesolutionTf,solution(-1.6°C)correspondstothetemperatureatwhichalinedrawnthroughthedatapointspertainingtorégime#1inter-sectsalinedrawnthroughthedatapointspertainingtorégime#2. Experiment5·Freezing-pointdepression5-12becompromisedificeformingintheCoCl2solutionshovesthethermometersomuchtoonesidethatittouchesthewallofthetesttube.Ifthishappens,youaremeasuringthetemperatureofthetesttubeandnotofthesolution.Topreventthisfromhappening,beforeyoubeginarunbesurethatthetipofthethermometerisdippingstraightdownintothemiddleoftheCoCl2solutionandnotpointingofftooneside.Ifyouseethethermometerbegintopointtoonesideduringarun,manuallyrepositionitsothatitispointingstraightdown.Afterthecompletionoftherun,removethetesttubefromthecoldbathandplaceitabeakerofwarmtapwateruntilthesolutionmelts.Monitorthetemperatureofthesolutionasitmelts:youwanttoremovethetubefromthewarmwaterbathwhenitreachesroomtemperature.ReusingthesameCoCl2solution,repeatthefreezing-pointdeterminationtwicemore:youwantdatafromthreeruns.Youdon'thavetoprepareanewsolution-justreusetheliquidinthebigtesttube.Beforeeachrun(1)besurethatthetempera-tureofthesolutionhasstabilizedaroundroomtemperature,thatis,donotbeginthenextrunwithhotorevenwarmsolu-tion;(2)checkthatthetemperatureofthecoldbathisbelow-15 °C.Ifitisnot,stirthesalt-icemixturewithaspatula;ifstirringdoesnotbringth etemperaturebackdownbelow-15 °C,disposeofthecoldbathinthesinkandprepareafreshcoldbath.DeterminationofthefreezingpointofaconcentratedsaltsolutionAfteryourthirdrunusingthedilutesaltsolution,youwillin-creasetheconcentrationofCoCl2inthesolutionandrecordthefreezingpointoftheconcentratedsolutionthreetimesasbefore.AddenoughCoCl2(s)sothatthetotalmassofCoCl2insolutionisabout3.5g,butrememberthatyoualreadyaddedabout1go fCoC l2soproceedaccordingly.Recordinyour notebookthemassyouweighouttotwodecimals,andaddtheCoCl2(s)tothesolutioninthetesttube.Placethetest tubecont ainingthesolutionintot hecoldbath.Immediatelyrecordthetemperatureofthesolutionuponimmersion(callthist=0 sec)andevery20 secthereafter.Youshouldonceagainobservethethreeseparaterégimesofcool- Experiment5·Freezing-pointdepression5-13ingbehaviormentionedearlier.Asbefore,youshouldideallycontinuetakingtime-versus-temperaturedatauntilyouhavepositivelyidentifiedthethirdrégimeoftemperaturedecreaseorfor10min.Afterthecompletionofeachrun,melttheCoCl2solutionbyplacingthetesttubeinabeakerofwarmtapwateruntilthesolutionreachesroomtemperature.Reusingthesamesolution,repeatthefreezing-pointdeterminationtwicemore:youwantdatafromthreeruns.Beforeeachrunmakesurethatthetemperatureofthecoldbathisbelow-15 °C:ifitisnot,takeappropriateactiontomakeitso.Clean-upThelethaldoseofCoCl2fora63-kg(150-lb)personisabout30gandsub-lethaldosescancauseheartproblems:wedon'twanttodumpCoCl2downthedrain.Whenyouhavefinishedallexperimentalwork,disposeoftheCoCl2solutioninahaz-ardouswastecontainer,rinseoutthebigtesttubewithwater,alsodisposingoftherinsesinahazardouswastecontainer.Tosstheinsulatingblanketaroundthe600-mLbeakerinthetrashanddisposeoftheice-saltcoldbathinthesink.NaCl(s)
H 2 O → ⎯ ⎯ ⎯ ⎯ ⎯ Na (aq)+Cl (aq) 4.00gNaCl
molNaCl 58.44gNaCl
473mLH
2 O 1.00gH
2 O mLH 2 O kgH 2 O 1000gH
2 O =0.145molNaCl/kgH 2 O 1.86°C⋅kgH
2 O molNaCl 0.145molNaCl
kgH 2 O =-0.538°C 1.86°C⋅kgH
2 O molNaCl 0.44molCuSO
4 kgH 2 O =-1.64°C i=- T f,solution K f,H2O m -0.82°C 1.86°C⋅kgH
2 O molCuSO 4 0.44molCuSO
4 kgH 2 O =1.0