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EXPERIMENT30A4:HYDRATE
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Describethedifferencesbetweenananhydrousandhydratecompound.
2) Calculatethenumberofmolesofwaterinahydratedcompoundbasedon
experimentaldata.
Introduction
Severalinorganiccompoundshavewatermoleculesbondedtothemetalion.The
natureofthebondingbetweenthemetalionandwatermoleculeswillbediscussed
indetailinalatertopic.Theinorganiccompoundwithboundwatermoleculesis
calledahydrate.Thenumberofwatermoleculesboundtothemetalionvariesfrom
compoundtocompound.Someexamplesofhydratedsaltsaregivenbelow:
CuSO4!5H2O Copper(II)sulfatepentahydrate
CaSO4!2H2O Calciumsulfatedihydrate
MgSO4!7H2O Magnesiumsulfateheptahydrate
ZnSO4!7H2O Zincsulfateheptahydrate
HydratedCompound:Ahydratedcompoundisdefinedasonethatcontainsafixed
numberofwatermoleculeschemicallycombinedwiththemetalion.
AnhydrousCompound:Ananhydrouscompoundisdefinedasoneinwhichthe
molecule(s)ofwaterofhydrationhasbeenremoved.
Theanhydrousandhydratedformsofaparticularcompoundmayexhibitdifferent
properties.Forinstance,anhydrouscopper(II)sulfateiswhiteincolorwhereasthe
hydratedform,copper(II)sulfatepentahydrate,isblueincolor.
Severalcompounds,suchassodiumhydroxide,zincchloride,sodiumchloride,etc.,
canabsorbwaterfromtheatmosphere.Suchcompoundsaresaidtobehygroscopic.
Hydratedcompoundscanalsobehygroscopic,butonlythosewaterschemically
combinedwiththemetalionareconsideredpartofthehydrate;asubstanceisnot
necessarilyahydratesimplybecauseitcontainswater.
Somehygroscopicsubstancessuchassodiumhydroxideareabletoabsorbenough
watersoastocompletelybesolubilizedbytheabsorbedwater.Thispropertyis
knownasdeliquescence.
Adesiccantisachemicalsubstancethatishygroscopic;andduetoitsabilityto
absorbwateritisusedtomaintainadryenvironmentorabsorbmoisture.Packets
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ofdesiccants,suchasthosemadewithsilica,areoftenusedtoabsorbmoisturefrom
clothingandpillbottles.Desiccantsarealsousedindesiccatorsinalaboratoryto
keepchemicals“dry”.
ExperimentalDesign
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Inthisexperiment,youwillworkfirstworkwithcopper(II)sulfatepentahydrate
andconfirmthatthereareindeedfivemolesofwaterassociatedwitheachmoleof
copper(II)sulfate.Followingthis,youwillbeprovidedwithoneofthreehydrated
compounds:CaSO4.xH2O,MgSO4.yH2O,ZnSO4.zH2O.Thegoaloftheexperimentisto
determinethenumberofmolesofwaterpermoleoftheanhydrouscompoundin
thegivenunknown.
Theexperimentaldesignisbasedonthefactthatheatingthehydratedcompound
caneliminatethewatersofhydration.Thedifferenceinthemassbetweenthe
hydratedandanhydrouscompoundwillbethemassofthewater.Oncethemassof
theanhydrouscompoundandthewaterareknown,themolesoftheanhydrous
compoundandwatercanbecalculatedtoobtaintheformulaofthehydrated
compound.
ReagentsandSupplies
CuSO4!5H2OandoneofCaSO4.xH2O,MgSO4.yH2O,ZnSO4.zH2O
(SeepostedMaterialSafetyDataSheets)
Micro-crucibles
Procedure
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PART1:ANALYSISOFCOPPER(II)SULFATEPENTAHYDRATE
1. Heatanemptycrucibleandbringittoaconstantmass(±0.0010g).Inorderto
dothis,recordthemassoftheemptycrucible.Thenheatthecruciblefor3-4
minutes.Oncethecrucibleiscool,recorditsmassagain.Repeattheheatingcooling-weighingprocessuntilthemassrecordedisthesameasthatforthe
previousmeasurement.
2. Addsomecopper(II)sulfatepentahydratetothemicro-crucible(scoopasmall
amountwithaspatula,approximately0.1000grams).Measurethemassofthe
micro-cruciblewiththesolid.
3. Heatthemicro-crucibleoveraBunsenburner.Whenthewaterofhydrationhas
beenevaporated,thecolorofthesolidshouldchangefrombluetograyish-white.
4. Coolthemicro-crucibletoroomtemperatureandmeasurethemassofthe
micro-cruciblewiththeanhydrouscompound.
5. Inordertoensurethatallthewaterofhydrationhasbeenevaporatedfromthe
solid,themicro-cruciblecontainingtheanhydroussolidshouldbeheatedover
theflameonceagain.
6. Onceagain,coolthemicro-crucibleandmeasurethemassofthemicro-crucible
containingtheanhydrouscompound.
7. Theheating-cooling-weighingofthemicro-cruciblewiththeanhydrous
compoundmustberepeatedafewtimes(threetofourtimes)untilthemassof
themicro-cruciblecontainingtheanhydrouscompoundisaconstantbetween
thelasttwomeasurements(±0.0010g).
8. Discardthecontentsofthemicro-crucibleintheappropriatewastedisposal
container.
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PART2:ANALYSISOFUNKNOWNHYDRATE
1. Heatanemptycrucibleandbringittoaconstantmass(±0.0010g).Inorderto
dothis,recordthemassoftheemptycrucible.Thenheatthecruciblefor3-4
minutes.Oncethecrucibleiscool,recorditsmassagain.Repeattheheatingcooling-weighingprocessuntilthemassrecordedisthesameasthatforthe
previousmeasurement.
2. Addsomeunknownhydratetothemicro-crucible(scoopasmallamountwitha
spatula,approximately0.1000grams).Measurethemassofthemicro-crucible
withthesolid.
3. Heatthemicro-crucibleoveraBunsenburner.Continuetheheatingprocessfor
aboutfiveminutes.
4. Coolthemicro-crucibletoroomtemperatureandmeasurethemassofthe
micro-cruciblewiththeanhydrouscompound.
5. Inordertoensurethatallthewaterofhydrationhasbeenevaporatedfromthe
solid,themicro-cruciblecontainingtheanhydroussolidshouldbeheatedover
theflameonceagain.
6. Onceagain,coolthemicro-crucibleandmeasurethemassofthetesttube
containingtheanhydrouscompound.
7. Theheating-cooling-weighingofthemicro-cruciblewiththeanhydrous
compoundmustberepeatedafewtimes(threetofourtimes)untilthemassof
themicro-cruciblecontainingtheanhydrouscompoundisaconstantbetween
thelasttwomeasurements(±0.0010g).
8. Discardthecontentsofthemicro-crucibleintheappropriatewastedisposal
container.
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DataTable
PART1:ANALYSISOFCOPPER(II)SULFATEPENTAHYDRATE
Constantmassofemptymicro-crucible
(grams)
Massofmicro-crucible+hydrate(grams)
Massofmicro-crucible+anhydrous
compound(followingfirstheating)(grams)
Massofmicro-crucible+anhydrous
compound(followingsecondheating)
(grams)
Massofmicro-crucible+anhydrous
compound(followingthirdheating)(grams)
Massofmicro-crucible+anhydrous
compound(followingfourthheating)
(grams)
Trial1
Trial2
Trial3
Trial1
Trial2
Trial3
PART2:ANALYSISOFUNKNOWNHYDRATE
Constantmassofemptymicro-crucible
(grams)
Massofmicro-crucible+hydrate(grams)
Massofmicro-crucible+anhydrous
compound(followingfirstheating)(grams)
Massofmicro-crucible+anhydrous
compound(followingsecondheating)
(grams)
Massofmicro-crucible+anhydrous
compound(followingthirdheating)(grams)
Massofmicro-crucible+anhydrous
compound(followingfourthheating)
(grams)
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Calculations
PART1:ANALYSISOFCOPPER(II)SULFATEPENTAHYDRATE
Trial1
Massofemptymicro-crucible(grams)
Massofmicro-crucible+hydrate
(grams)
Massofhydrate(grams)
Constantmassofmicro-crucible+
anhydrouscompound
(followingfinalheating)(grams)
Massofanhydrouscompound(grams)
Massofwater(grams)
Molesofanhydrouscompound
Molesofwater
Formulaofhydratedcompound
Trial2
Trial3
PART2:ANALYSISOFUNKNOWNHYDRATE
A. AssumethattheanhydrouscompoundisCaSO4
Trial1
Massofemptymicro-crucible(grams)
Massofmicro-crucible+hydrate
(grams)
Massofhydrate(grams)
Constantmassofmicro-crucible+
anhydrouscompound
(followingfinalheating)(grams)
Massofanhydrouscompound(grams)
Massofwater(grams)
Molesofanhydrouscompound
Molesofwater
Formulaofhydratedcompound
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Trial2
Trial3
B. AssumethattheanhydrouscompoundisMgSO4
Trial1
Massofemptymicro-crucible(grams)
Massofmicro-crucible+hydrate
(grams)
Massofhydrate(grams)
Constantmassofmicro-crucible+
anhydrouscompound
(followingfinalheating)(grams)
Massofanhydrouscompound(grams)
Massofwater(grams)
Molesofanhydrouscompound
Molesofwater
Formulaofhydratedcompound
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Trial2
Trial3
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C. AssumethattheanhydrouscompoundisZnSO4
Trial1
Massofemptymicro-crucible(grams)
Massofmicro-crucible+hydrate
(grams)
Massofhydrate(grams)
Constantmassofmicro-crucible+
anhydrouscompound
(followingfinalheating)(grams)
Massofanhydrouscompound(grams)
Massofwater(grams)
Molesofanhydrouscompound
Molesofwater
Formulaofhydratedcompound
Trial2
Trial3
Results
Theidentityoftheunknownprovidedis:______________________________________________
Theformulaofthehydratedunknowncompoundis:_________________________________