solutions colligative properties - sci yeung . c o m properties colligative properties are...

SolutionsColligativeProperties

ColligativeProperties

Colligativepropertiesarepropertiesofsolutionsthatdependontheconcentrationofsolutemoleculesorionsbutnotonthe

identityofthesolute.

Topics

• Howtemperatureaffectssolubilityofgases• Howpressureaffectssolubilityofgases• Freezing-pointdepressionvs boiling-pointelevation

• Representationsofconcentrations• Molarityandmakingsolutions• Dilutions(dilutingsolutions)

Howtemperatureaffectssolubilityofgases

Basedonthesolubilitycurve,Howdoestemperatureaffectsolubilityofsolids?

Increases

Howdoestemperatureaffectsolubilityofgases?

Decreases

Whydoessolubilityofgasesdecreases?

• Increasetemperature=increaseinenergyofgasparticles

• Increaseofenergyallowstheparticlestoescapemorereadily

• Examplefromreallife?– LeavingacanofCokeinthesun

– Hotweather(Globalwarming)leavinglessdissolvedoxygeninlakescausingdeath

Howpressureaffectssolubilityofgases

• Gasparticlesdissolvedinsolutionareindynamicequilibriumwiththegasesoutsideofthesolution.

• Ifpressureoutsideofsolutionisdecreased,thenthepressurefromthegasesinsidethesolution canescapemorereadilyandviceversa.

@ Equilibrium Lowpressure

Boilingpointelevation

• Whydoweaddsaltintoourwaterwhenwecookpasta?

BoilingpointelevationWhatisit?• Thenormalboilingpointofasolutionisraisedrequiringmoreenergyforittoboil.

Why?• Whensoluteisaddedtoasolution,themixtureofsoluteandthesolutionwillcausethevapourpressure tobelowered

• Causingthesolutiontoenterthegasphasemoredifficult,thusrequiringmoreenergy.

NormalBPofwater@100C

BPElevatedduetosalt

Purewateratregularboilingpoint

Impurities insolution

Theimpurities causesadisturbance insolution allowinglesswatertobeonthesurface,whichlowersthevapour pressure

LowerVapour pressuremeanslesspressuretoenterthegasphase,soinorder forittogo intogasphase,moreenergy isneeded!MOREHEAT!B.PisElevated!

Results–• Temperatureisraisedbeforeasolutionstartstoboil.

• Therefore,pastacooksfasterbecausethewaterishotterthan100C

FreezingpointdepressionWhatisit?• Thenormalfreezingpointis

lowered andrequiringMORE energyforittobetakenoutfromtheliquidtofreeze.

• Insteadoffreezingat0Cforwater,itmayfreezeat-9C

Why?• Whensoluteisaddedtosolution,

theimpuritiesmakeitmoredifficultforthesolutiontoentersolidphase(tocrystalize)

• Therefore,alowertemperaturemustbeachieveinorderforthesolutiontofreeze.

NormalFreezingpointofwaterat0°C

Insteadoffreezingat0°C,itfreezeslowerthanthenormalmarklike-8°C.

Thismeansthatitrequireslessenergytobetakenaway

Freezingpointdepression

NormalFreezingpointofwaterat0°C

Insteadoffreezingat0°C,itfreezeslowerthanthenormalmarklike-8°C.

Thismeansthatitrequireslessenergytobetakenaway

Soatthatpoint,thewaterisstillinliquid form,eventhough itisat-8°C.

Reallifeexample• Saltonroads– Thesaltmeltstheice,andcreatesabrine solution(saltwatermixture)

– Thesaltandsolutionmakesitmoredifficultforthesolutiontofreeze,sothesolutionrequiresalower temperatureforittofreezeagain.

Differentde-icers• De-icers arenormallyCaCl2 insteadofNaCl.

• CaCl2 worksbetterbecauseitdissociatesinto3atomsinsteadof2ofNaCl.

• The3atomsdisruptsthesolutionmorecausingthefreezingpointtolowermorethanNaCl.

NaCl

CaCl2

Na+ Cl-

Cl- Cl-Ca+2

Moredisturbance!

Differentwaysofrepresentingconcentration

• %weight-weight(%w/w)• %weight- volume(%w/v)

• %volume/volume(%v/v)

• partspermillion(ppm)• partsperbillion(ppb)• gramsperlitre (g/L)

Example

• GoldBondhas1%w/wofPramoxineHydrochloride

• Thetotalweightofthecreamis28g,howmuchofPramoxin Hydrochloridewasused?

• Calculate1%of28g.• 0.01x28=0.28gofPramoxin Hydrochlorideor280mg

HydrocortisoneCream

• Has0.5%w/wofHydrocortisone• Howmuchofhydrocortisonewasusedina15gbottle

• Solution• 0.5%x15g=0.075gor75mgofhydrocoritisone

Makingsolutions

• Inorderforasolutiontobeaccuratelycreated,molesmustbeinvolved

• Sincesolutionsaresolutesdissolvedinsolvent,bothmolesandvolumemustbeincluded.

• Molarity!

Molarityandcalculations

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

Example 1. Whatisthemolarityofa5.00litersolutionthatwasmadewith

10.0molesof KBr ?

Example2. A250mlsolutionismadewith0.50molesofNaCl. Whatisthe

Molarityofthesolution?

Example 3: Whatwouldbethevolumeofa2.00M(moles/L)solution

madewith6.00molesofLiF?

Example 4. HowmanymolesofCaCl2wouldbeusedinthemaking0.500Lofa5.0Msolution?Howmanygrams

ofCaCl2 wereused?

Example5. Whatisthevolumeof3.0MsolutionofNaCl madewith526g

ofsolute?

Howwouldyoupreparea6.0MNaOHsolutionina1Lbottle?

w• Calculatemolesrequiredtofillthe1Lbottle.• 6.0M(mol/L)x1L=6molesofNaOH• 6molesx(40.01g/mol)molarmassofNaOH =240gofNaOH required.