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SolutionsColligativeProperties
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ColligativeProperties
Colligativepropertiesarepropertiesofsolutionsthatdependontheconcentrationofsolutemoleculesorionsbutnotonthe
identityofthesolute.
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Topics
• Howtemperatureaffectssolubilityofgases• Howpressureaffectssolubilityofgases• Freezing-pointdepressionvs boiling-pointelevation
• Representationsofconcentrations• Molarityandmakingsolutions• Dilutions(dilutingsolutions)
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Howtemperatureaffectssolubilityofgases
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Basedonthesolubilitycurve,Howdoestemperatureaffectsolubilityofsolids?
Increases
Howdoestemperatureaffectsolubilityofgases?
Decreases
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Whydoessolubilityofgasesdecreases?
• Increasetemperature=increaseinenergyofgasparticles
• Increaseofenergyallowstheparticlestoescapemorereadily
• Examplefromreallife?– LeavingacanofCokeinthesun
– Hotweather(Globalwarming)leavinglessdissolvedoxygeninlakescausingdeath
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Howpressureaffectssolubilityofgases
• Gasparticlesdissolvedinsolutionareindynamicequilibriumwiththegasesoutsideofthesolution.
• Ifpressureoutsideofsolutionisdecreased,thenthepressurefromthegasesinsidethesolution canescapemorereadilyandviceversa.
@ Equilibrium Lowpressure
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Boilingpointelevation
• Whydoweaddsaltintoourwaterwhenwecookpasta?
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BoilingpointelevationWhatisit?• Thenormalboilingpointofasolutionisraisedrequiringmoreenergyforittoboil.
Why?• Whensoluteisaddedtoasolution,themixtureofsoluteandthesolutionwillcausethevapourpressure tobelowered
• Causingthesolutiontoenterthegasphasemoredifficult,thusrequiringmoreenergy.
NormalBPofwater@100C
BPElevatedduetosalt
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Purewateratregularboilingpoint
Impurities insolution
Theimpurities causesadisturbance insolution allowinglesswatertobeonthesurface,whichlowersthevapour pressure
LowerVapour pressuremeanslesspressuretoenterthegasphase,soinorder forittogo intogasphase,moreenergy isneeded!MOREHEAT!B.PisElevated!
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Results–• Temperatureisraisedbeforeasolutionstartstoboil.
• Therefore,pastacooksfasterbecausethewaterishotterthan100C
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FreezingpointdepressionWhatisit?• Thenormalfreezingpointis
lowered andrequiringMORE energyforittobetakenoutfromtheliquidtofreeze.
• Insteadoffreezingat0Cforwater,itmayfreezeat-9C
Why?• Whensoluteisaddedtosolution,
theimpuritiesmakeitmoredifficultforthesolutiontoentersolidphase(tocrystalize)
• Therefore,alowertemperaturemustbeachieveinorderforthesolutiontofreeze.
NormalFreezingpointofwaterat0°C
Insteadoffreezingat0°C,itfreezeslowerthanthenormalmarklike-8°C.
Thismeansthatitrequireslessenergytobetakenaway
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Freezingpointdepression
NormalFreezingpointofwaterat0°C
Insteadoffreezingat0°C,itfreezeslowerthanthenormalmarklike-8°C.
Thismeansthatitrequireslessenergytobetakenaway
Soatthatpoint,thewaterisstillinliquid form,eventhough itisat-8°C.
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Reallifeexample• Saltonroads– Thesaltmeltstheice,andcreatesabrine solution(saltwatermixture)
– Thesaltandsolutionmakesitmoredifficultforthesolutiontofreeze,sothesolutionrequiresalower temperatureforittofreezeagain.
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Differentde-icers• De-icers arenormallyCaCl2 insteadofNaCl.
• CaCl2 worksbetterbecauseitdissociatesinto3atomsinsteadof2ofNaCl.
• The3atomsdisruptsthesolutionmorecausingthefreezingpointtolowermorethanNaCl.
NaCl
CaCl2
Na+ Cl-
Cl- Cl-Ca+2
Moredisturbance!
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Differentwaysofrepresentingconcentration
• %weight-weight(%w/w)• %weight- volume(%w/v)
• %volume/volume(%v/v)
• partspermillion(ppm)• partsperbillion(ppb)• gramsperlitre (g/L)
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Example
• GoldBondhas1%w/wofPramoxineHydrochloride
• Thetotalweightofthecreamis28g,howmuchofPramoxin Hydrochloridewasused?
• Calculate1%of28g.• 0.01x28=0.28gofPramoxin Hydrochlorideor280mg
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HydrocortisoneCream
• Has0.5%w/wofHydrocortisone• Howmuchofhydrocortisonewasusedina15gbottle
• Solution• 0.5%x15g=0.075gor75mgofhydrocoritisone
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Makingsolutions
• Inorderforasolutiontobeaccuratelycreated,molesmustbeinvolved
• Sincesolutionsaresolutesdissolvedinsolvent,bothmolesandvolumemustbeincluded.
• Molarity!
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Molarityandcalculations
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Molarity
• Molarity is a unit of concentration defined as the number of moles of solute dissolved in one litre of solution.
• Molarity=molesofsolute/1Litre• Example:ifyouhave0.5molesandyouwanttofindthemolarity.Itwouldbe
– 0.5moles/1L=.5M(mol/L)
• MolaritycansometimesbedenotedMformolarity
• Unitsformolarity=moles/litre orM
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Example 1. Whatisthemolarityofa5.00litersolutionthatwasmadewith
10.0molesof KBr ?
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Example2. A250mlsolutionismadewith0.50molesofNaCl. Whatisthe
Molarityofthesolution?
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Example 3: Whatwouldbethevolumeofa2.00M(moles/L)solution
madewith6.00molesofLiF?
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Example 4. HowmanymolesofCaCl2wouldbeusedinthemaking0.500Lofa5.0Msolution?Howmanygrams
ofCaCl2 wereused?
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Example5. Whatisthevolumeof3.0MsolutionofNaCl madewith526g
ofsolute?
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Howwouldyoupreparea6.0MNaOHsolutionina1Lbottle?
w• Calculatemolesrequiredtofillthe1Lbottle.• 6.0M(mol/L)x1L=6molesofNaOH• 6molesx(40.01g/mol)molarmassofNaOH =240gofNaOH required.