英文实验油膜法测阿伏伽德罗常数

ESTIMATIONOFAVOGADRO’SNUMBERThisexperimentisdesignedtoextendyourmeasurementcapabilitiesandtodeterminethevalueoftheconstantAvogadro’snumber.Inmanysituationswherequantitativemeasurementsareneeded,directmeasurementsuchaslayingarulernexttotheobjectisnotpossiblewhentheobjectistoolargeortoosmall.Insuchsituationsacommonprocedureistousesomemethodofindirectmeasurement.ThedeterminationofAvogadro’snumberisanexamplewherethisindirectmethodisneededbecauseitsmagnitudeismuchtoolargetodeterminebydirectcountingorothermethodsavailableinanintroductorylevellaboratory.IfoneconsiderstheunitsforAvogadro’snumber,mole-1,itisthenumberofanyitempermole.Onepossibleitemismoleculespermolewhichgivestheunits“moleculesmole-1”forthisconstant.Theseunitsmayalsobeexpressedintheformatmolecules/mole.ItispossibletodeterminethevalueofAvogadro’snumberfromapropertysuchasthemassorvolumeofasystemwhichmaybeexperimentallydeterminedforonemoleculeaswellasonemoleofthemolecules.Whenthisispossible,thevalueofAvogadro’snumberiscalculatedbyobtainingtheratioofthispropertyonamolebasistothatsamepropertyonamoleculebasis.Forexample,ifthemassofonemoleculeofsubstanceAismAandthemassofamoleofAisMAthevalueoftheratio(No)iscalculatedbytheexpression:No=MA/mA=(grams/mole)/(grams/molecule)=molecules/mole=6.02x1023molecules/moleInthisexperiment,thepropertymeasuredisthevolumeinsteadofthemassandthesubstancethatwillbeusedisOleicAcid(C18H34O2).ThevalueofAvogadro’snumberistobedeterminedbyobtainingtheratioofthevolumepermoleofOleicAcid(VA)tothevolumepermoleculeoftheacid(vA)asshownbelow.No=VA/vA=(cm3/mole)/(cm3/molecule)=molecules/mole=Avogadro’snumberThevolumeofonemoleofoleicacid,VA,isdeterminedbydividingthemolarmassbythedensitywhichis0.873g/cm3(VA=MA/ρ).ThemolarmassofOleicAcidisobtainedfromthemolecularformulagivenaboveandtheatomicweightsoftheelementsingrams.Theuseofindirectmeasurementcomesintouseindeterminingthevolumeofonemoleculeofoleicacid.WhenOleicAcidisplacedonthesurfaceofwatertheacidspreadsoutonthesurfaceinafilmonemoleculethick.Theacidendofthemolecule,–CO2H,issolubleinwaterbuttherestofthemoleculeisnot.Thiscausethemoleculestoorientthemselvesverticallywithrespecttothewatersurfaceformingamono-molecularfilmwithathicknessequaltothelengthofonemolecule.Fromageneralideaoftheshapeofthemolecule,longandnarrow,itispossibletoestimatethevolumeofthemoleculeasthesizeofacylinderorrectangularboxthatthismoleculewouldjustfitinto.Thus,themolecularvolumeisapproximatedasthevolumeofthecontainerrequiredtoholdonemolecule.ThevalueofAvogadro’snumberobtainedwilldependonhowwellthevolumeismodeledbytheshapeofthecontainerchosenandhowwellonecandeterminethethicknessoftheOleicAcidfilmonthesurfaceofthewater.Sincethethicknessofthefilmwillbedeterminedbytheareaandthevolumeofacidused,itisnecessarytoknowthevolumeofacidthatisplaceonthesurface.However,ifpureOleicAcidwasusedthefilmwouldbetoobigtomeasureinthelaboratory.Theacidisdissolvedinmethylalcoholandthissolutionisdroppedonthesurfaceofthewaterfromaneyedropper.ThealcoholissolubleinthewaterandgoesintosolutionleavingtheOleicAcidonthesurface.Theconcentrationoftheacidis0.50%byvolumeinmethylalcohol.Thevolumeoftheacidusedineachtrialistherefore0.0050*Vdropused.Theacidwillformacircularfilmonthesurfaceanditsareamaybedeterminebymeasuringthediameterofthecircle(D)andusingthestandardequationfortheareaofacircleA=πR2whereRistheradiusofthesurfacefilm(R=D/2).Thethicknessofthefilmisobtainedbydividingthevolumeofacidinonedropbytheareacalculatedforthecircularfilmthisdropmakesonthewater[t=VA/A=VA/πR2]sincetheshapeofthelayerofacidisapproximatelyanextremelyshortcylinder.VisthevolumeoftheOleicacidinonedropandRistheradiusofthesurfacefilmofOleicAcid.Thevolumeofthemoleculemaybereasonablyapproximatedbytwomodels,acylinderandarectangularbox,asshownbelow:Thevalueofthelengthofthemoleculeisthethicknessofthefilmtforbothmodels.Theradius,r,iscalculatedast/12forthecylinderandtheedgeist/6fortherectangularboxmodel.YouwillcalculateAvogadro’snumberwithbothmodelsandcomparethecalculatedvalueswiththeacceptedvalueof6.02x1023molecules/mole.Itisalsopossibletoobtainanapproximatevaluefortheradiusofacarbonatomfromthelengthofthemolecule.Oleicacidiseighteencarbonatomslongandthislengthis,therefore,approximately18timesthediameterofthecarbonatom.Thisgivestheradiusofthecarbonatomast/36sincethediameteristwicetheradius.PROCEDURECalibrationoftheeyedropper:SinceyouwillbeplacingtheOleicAcidsolutiononthewaterdropwiseitisnecessarytoknowthevolumeofasingledrop.Thesolutionismostlymethanolsothebestliquidtousetocalibratethedropperisthesolventmethanol.Todothis,countthenumberofdropsofmethanolnecessarytofilla10mlgraduatedcylindertobetween4and5mlandreadthevolumetransferred.Thevolumemeasuredisdividedbythenumberofdropscountedtogivethevolumeperdrop.Thecalibrationcountshouldbedonethreetimeoruntilyouobtainthreecalibrationvaluesthatagreetowithin5%.Differentvolumeamountsshouldbeusedforeachtrialbutatleast4mlshouldbeused.Besuretoholdthedropperasnearlyverticalaspossiblewhensqueezingoutthedropstoensurethedropsareofconsistentsize.FormationoftheOleicAcidfi