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EXPERIMENTC1:FREEZINGPOINTDEPRESSION
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Relatefreezingpointdepressiontomolalityofthesolution.
2) Measurethefreezingpointdepressionconstantforwater.
Introduction
Propertiesofasolutionthatonlydependontheamountofsoluteinthesolutionand
areindependentofthechemicalidentityofthesolutearereferredtoasColligative
Properties.Examplesofcolligativepropertiesinclude:1)boilingpointelevation2)
freezingpointdepression3)loweringofvaporpressureand4)osmoticpressure.
Inthisexperimentthefreezingpointdepressionofwaterduetothepresenceofa
solutewillbestudied.
Whensolutessuchassaltorglucoseareaddedtowater,thefreezingpointofwater
decreases.Theextenttowhichthefreezingpointisloweredordepresseddepends
ontheamountofthesolutethatisadded.Theamountofsoluteinthesolutionis
measuredintermsofmolality.Molality(m)isdefinedasthemolesofsoluteper
kilogramofthesolvent.Molalityisusedinsteadofmolarityintheseinstances,asthe
volumeofthesolventchangesslightlywithvariationsintemperaturewhilemas
doesnot.Thisallowsforconsistencyoverarangeoftemperatureswithout
additionalcalculationstoaccountforthermalexpansionofsolvents.
Thereforethedepressioninthefreezingpoint,ΔTfisproportionaltothemolality,m,
ofthesolution.
ΔTfαm
ΔTf=Kf!m
IntheaboveequationKf,theproportionalityconstant,isreferredtoasthefreezing
pointdepressionconstant.Thefreezingpointdepressionconstantforwateris1.86
°C/m.Thisvaluewillbeconfirmedexperimentally.
ExperimentalDesign
Inthisexperiment,differentamountsofanon-volatileandnon-ionicsolute,
glycerol,willbeaddedtowater.Thefreezingpointofwaterwillbemeasuredfor
eachamountofglyceroladded.Aplotofthemolalityvs.thefreezingpoint
depressionwillbeusedtodeterminethefreezingpointdepressionconstant.
ReagentsandSupplies
Glycerol,rocksalt,thermometer
(SeepostedMaterialSafetyDataSheets)
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Procedure
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1. Placeanemptybeakerontheelectronicbalanceandtarethebalance.
2. Placealargeemptytesttubeinsidethebeakerinstep1andrecordthemass.
3. Addabout12gramsofdeionizedwatertothetesttubeinstep2andrecordthe
mass.
4. Addabout1gramofglycerolintothetesttubeinstep3andrecordthemass.
Useadroppertoaddtheglycerol.Glycerolishighlyviscousandmaynotflow
easilyinandoutofthedropper.Caremustbetakentoavoidglycerolspillson
thebalancepan.Incaseofspills,usepapertowelstoimmediatelycleanthe
glycerolanddiscardthepapertowelinthetrashcontainer.Makesurethatthe
glyceroliscompletelydissolvedbeforebeginningthecoolingprocess.
5. Repeatsteps1through4withthefollowingamountsofglycerol.
TestTube Water(grams) Glycerol(grams)
1
12
1.0
2
12
1.4
3
12
1.8
4
12
2.4
5
12
3.0
6
12
3.8
7
12
4.5
8
12
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6. Determinethefreezingpointofwaterineachofthetesttubeusingthefollowing
method.
a. Clampthetesttubecontainingthewater/glycerolsolution.
b. Prepareacoldwaterbathbycombiningwater,rocksalt,andiceinthe
followingproportionina600-mLbeaker:25%water,25%rocksalt,
and50%ice.Thetemperatureofthebathshouldbeatleast−8°C.
c. Placethetesttubeinsidecontainingthewater/glycerolsolutioninthe
coldwaterbath.Makesurethatthesolutioninsidethetesttubeis
completelysubmergedintotheicemixture.
d. Placeathermometerinsidethetesttubeandlightlystirthecontents,
takingcaretoavoidbreakingthetesttube.
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e. Whenthemixturebecomesathickslurry,removethetesttubefrom
theicebath.
f. Removethetesttubefromicebathandbeginstirringtomelttheice
crystals.Dipthetesttubeintoaroomtemperaturewaterbath
immediatelyinordertofacilitatemelting.Donotleavethetesttube
insidetheroomtemperaturebathasthismightleadtoartificiallyhigh
meltingpoints.
g. Stirthecontentsofthetesttubegently,usingthethermometer.
Recordthetemperatureatwhichthelastcrystalhasmelted.Thisis
themeltingpoint(NOTE:thisisthesameasthefreezingpoint).
7. Oncethefreezingpointsofthewaterinallthetesttubeshavebeenmeasured,
discardthesolutionsasdirectedbytheinstructor.
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DataTable
NOTE:Normalfreezingpointofwater=0°C
ΔTf=Measuredfreezingpint–Normalfreezingpoint
Test
MassofEmptyTest MassofTestTube MassofTestTube
Tube
Tube
+Water
+Water+Glycerol
(grams)
(grams)
(grams)
1
2
3
4
5
6
7
8
ΔTf(°C)
DataAnalysis
Massofwater=(Massoftesttube+water)–(Massofemptytesttube)
Massofglycerol=(Massoftesttube+water+glycerol)–(Masstesttube+water)
Massofsolution=Massofwater+Massofglycerol
Molarmassofglycerol=92.1g/mol
moles of glycerol
Molality =
Mass of solvent (kg)
Test Massof Massof Massof
Molesof Molality ΔTf
Tube water
water glycerol
glycerol
(°C)
(grams) (kg)
(grams)
1
2
3
4
5
6
7
8
6
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DataAnalysis
Plotagraphofmolality(x-axis)vs.ΔTf(y-axis).
Obtaintheequationofthebest-fitline.Besuretosetthey-intercepttozero.
TheslopeofthislineistheKf.
Theexperimentalvalueofthefreezingpointdepressionconstantforwater=
%errorinthemeasurement=