1
EXPERIMENTA6:CONDUCTIVITY
LearningOutcomes
Uponcompletionofthislab,thestudentwillbeableto:
1) Analyzedifferentsolutionsandclassifythemasbeingastrong
electrolyte,weakelectrolyte,oranon-electrolyte.
2) Measuretheconductivityofionicsolutionsanddeterminetheeffectof
molarconcentrationaswellasthenumberofionsinsolutiononthe
conductivity.
Introduction
Basedontheirabilitytoconductelectricity,solutionsmaybeclassifiedasbeing
strong,weak,ornon-electrolytes.Strongelectrolytesdissociatecompletelyinto
theircomponentionsandareasaresultstrongconductors.Weakelectrolytesonly
dissociatepartiallyintotheircomponentionsandareasaresultpoorconductors.
Non-electrolytes,ontheotherhand,donotdissociateintoionsandasaresultdo
notconductelectricity.
Solubleioniccompounds(suchasNaCl),andaqueoussolutionsofstrongacids(such
asHNO3)andstrongbases(suchasNaOH)areconsideredtobestrongelectrolytes.
Thedissociationequationsforthesestrongelectrolytesareshownbelow:
NaCl(aq)!Na+(aq)+Cl−(aq)
HNO3(aq)!H+(aq)+NO3−(aq)
NaOH(aq)!Na+(aq)+OH−(aq)
Ineachoftheabovecases,thearrowpointinginonedirectionimpliesthatevery
moleculeofthesubstanceontheleftdissociatesintoitsrespectiveions.For
instance,onemoleofNaCl(aq)woulddissociateintoonemoleofNa+(aq)andonemole
ofCl−(aq).
Weakacids(suchasHF)andweakbases(suchasNH3orNH4OH)areconsideredto
beweakelectrolytes.Thedissociationequationsfortheseweakelectrolytesare
shownbelow:
HF(aq)
H+(aq)+F−(aq)
NH4OH(aq)
NH4+(aq)+OH−(aq)
2
Ineachoftheabovecases,thearrowpointinginbothdirectionsimpliestwo
importantpoints:1)Notallthemoleculesoftheweakelectrolytedissociateto
releasetheirrespectiveions.Infact,amajorityoftheweakelectrolyteremainsas
intactmolecules.ForinstanceinthecaseofHF(aq),forevery10,000moleculesof
hydrogenfluoridedissolvedinwater,onlyONEdissociatesintoH+(aq)andF−(aq).2)
Theprocessisreversible.Thisisbecausethesesubstancesaremadeofcovalent
bondsratherthanionicbonds.
Molecularcompounds(suchasCH4,CO2etc.)areconsideredtobenon-electrolytes.
Sinceconductivityinaqueoussolutionsisapropertythatarisesduetothepresence
ofchargedspecies(cationsandanions),itislikelythatagreaterconcentrationof
ionsinthesolutionshouldresultingreaterconductivity.Thishypothesiscanbe
testedusingdifferentconcentrationsofaparticularsolution.Asthemolar
concentrationofthesolutionincreases,theconductivityshouldalsoincrease.An
extensionofthishypothesisisthat,whenoneexaminesthesamemolar
concentrationofdifferentsolutions,thesolutionthatgeneratesthegreatest
concentrationofdissolvedionswillhavethegreatestconductivity.
3
ExperimentalDesign
Conductivityofvarioussolutionswillbemeasuredusingaconductivityprobe.The
valueoftheconductivityisgiveninunitsofµS/cm-alargervalueindicatesgreater
conductivity.Inthefirstpartoftheexperiment,theconductivityofasetofsolutions
willbemeasuredandtheconductivityvalueswillbeusedtoclassifythesolutionas
beingastrongelectrolyte,weakelectrolyte,oranon-electrolyte.Inthesecondpart
oftheexperiment,theconductivityofdifferentmolarconcentrationsofNaCl(aq),
CaCl2(aq),andAlCl3(aq)willbemeasuredtodeterminetheeffectofconcentrationand
numberofionsinsolutiononconductivity.
ReagentsandSupplies
ReagentsetforPart1:solutionsofsodiumchloride,calciumchloride,aluminum
chloride,hydrochloricacid,phosphoricacid,aceticacid,boricacid,methanol,and
ethyleneglycol.Alsoobtaindeionizedwaterandtapwater.
ReagentsforPart2:0.05Mconcentrationsofsodiumchloride,calciumchloride,and
aluminumchloride.
(SeepostedMaterialSafetyDataSheets)
Vernierkitwithaconductivityprobe(fromstockroom),laptopcomputer(fromthe
lab)
Procedure
4
PART1:CLASSIFYTHEGIVENSOLUTIONASASTRONG/WEAK/NONELECTROLYTE
1. Obtainasetofreagentsincluding:0.05MNaCl,0.05MCaCl2,0.05MAlCl3,0.05
MHCl,0.05MH3PO4,0.05MCH3COOH,0.05MH3BO3,CH3OH,C2H6O2,tapwater,
deionizedwater.Eachlabbenchwillhaveasetofthesereagentstoshare.
2. Obtainavernierkitcontainingaconductivityprobefromthestockroom.Obtain
alaptopcomputerfromtheinstructor.
3. Connectthevernierkitandthelaptopcomputeraccordingto“Instructionsfor
ExperimentA6”(foundintheAppendix).Makesurethatthetoggleswitchisset
to0–20,000µS/cm.
4. Rinsetheconductivityprobewithdeionizedwaterandwipedrywithapaper
towel.
5. Placetheprobeintothefirstsolution.Recordtheconductivityofthesolution(it
maybenecessarytowaitafewsecondsforthevaluetostabilize).
6. Onceagain,rinsetheconductivityprobewithdeionizedwaterandwipedrywith
apapertowel.
7. Placetheprobeinthenextsolution.Repeattheprocessuntiltheconductivityof
allthereagentshavebeenmeasured.
PART2:DETERMINETHEEFFECTOFMOLARCONCENTRATIONANDNUMBEROFIONSINASOLUTION
ONTHECONDUCTIVITYOFTHESOLUTION
1. Obtainaround10-mleachof0.05MNaCl,CaCl2,andAlCl3.
2. Obtainaburettestand,buretteclampandsetupamicroburette.
3. Rinsetheburettewithdeionizedwaterandthenconditiontheburettewith0.05
MNaClandfilltheburettewith0.05MNaCl.
4. Addexactly35.0mlofdeionizedwatertoa125-mlErlenmeyerflask.
5. Obtainavernierkitcontainingaconductivityprobefromthestockroom.Obtain
alaptopcomputerfromtheinstructor.
6. Connectthevernierkitandthelaptopcomputeraccordingto“Instructionsfor
ExperimentA6”(foundintheAppendix).Makesurethatthetoggleswitchisset
to0–2000µS/cm.
5
7. Rinsetheconductivityprobewithdeionizedwaterandwipedry.
8. MeasuretheconductivityofthedeionizedwaterintheErlenmeyerflask(from
step4)andrecordthevalue.
9. Add0.1mlof0.05MNaClfromthemicroburetteintotheErlenmeyerflask.Swirl
theflasktothoroughlymixthecontentsoftheflask.
10. Measuretheconductivityofthesolutioninstep9andrecordthevalue.
11. Addanother0.1mlof0.05MNaClfromthemicroburetteintothesame
Erlenmeyerflask(fromstep9).Swirltheflasktothoroughlymixthecontentsof
theflask.
12. Measuretheconductivityofthesolutioninstep11andrecordthevalue.
13. Continueadding0.1mlof0.05MNaClfromthemicroburetteintothe
Erlenmeyerflask,swirltomix,measuretheconductivity,andrecordthevalue
foranadditionalSIXmoretimes.
14. Rinsetheconductivityprobewithdeionizedwateranddrywithapapertowel.
15. DiscardtheNaClfromtheburetteandtheErlenmeyerflaskintoawastebeaker.
16. Repeatallofthesamestepswith0.05MCaCl2and0.05MAlCl3.
DataTable
PART1:CLASSIFYTHEGIVENSOLUTIONASASTRONG/WEAK/NONELECTROLYTE
Solution
Conductivity(µS/cm)
NaCl
CaCl2
AlCl3
HCl
H3PO4
CH3COOH
H3BO3
CH3OH
C2H6O2
H2O(tap)
H2O(deionized)
6
7
PART2:DETERMINETHEEFFECTOFMOLARCONCENTRATIONANDNUMBEROFIONSINASOLUTION
ONTHECONDUCTIVITYOFTHESOLUTION
Conductivity(µS/cm)
Trial Volume
NaCl
CaCl2
AlCl3
(ml)
1
0.0
2
0.1
3
0.2
4
0.3
5
0.4
6
0.5
7
0.6
8
0.7
9
0.8
DataAnalysis
PART1:CLASSIFYTHEGIVENSOLUTIONASASTRONG/WEAK/NONELECTROLYTE
1. Basedonthedatalistallthestrongelectrolytesinthegivensetofreagents.
2. Foreachstrongelectrolytelistedabove,writethedissociationequation.
3. Basedonthedata,listalltheweakelectrolytesinthegivensetofreagents.
4. Foreachweakelectrolytelistedabove,writethedissociationequation.
5. Basedonthedata,listallthenon-electrolytesinthegivensetofreagents.
8
9
PART2:DETERMINETHEEFFECTOFMOLARCONCENTRATIONANDNUMBEROFIONSINASOLUTION
ONTHECONDUCTIVITYOFTHESOLUTION
1. Calculatetheconcentrationoftheelectrolyteineachtrial.(NOTE:Trial1hasno
electrolyteandhasonlydeionizedwater,sotheconcentrationoftheelectrolyte
inthiscaseis0.00M).Calculationsfortrial2areshownbelowasasample.
Trial2:
Initialmolarity(M1)=0.050M Initialvolume(V1)=0.100ml
Finalmolarity(M2)=? Finalvolume(V2)=35.1ml
M1V1=M2V2
0.050M×0.100ml=M2×35.1ml M2=0.00014M
Trial3:
Trial4:
Trial5:
Trial6:
Trial7:
Trial8:
Trial9:
10
2. Entertheconductivityvaluesasafunctionofconcentrationoftheelectrolytefor
eachtrialinthefollowingtable.
Conductivity(µS/cm)
Trial Concentration,M
NaCl
CaCl2
AlCl3
1
0.00
2
3
4
5
6
7
8
9
3. Plotagraphoftheconcentrationoftheelectrolyte(x-axis)vs.theconductivityof
theelectrolyte(y-axis).Plotallthreelinesonthesamegraphandfiteachlineto
theequationofastraightlineandobtaintheslopeofeachline.Attachthegraph
toyourworksheets.
11
4. Whataretheslopesofthelinescorrespondingtoeachelectrolytesolutioninthe
abovegraph?
Electrolyte Slope
NaCl
CaCl2
AlCl3
5. Writethedissociationequationforeachoftheelectrolytes.
NaCl(aq)!
CaCl2(aq)!
AlCl3(aq)!
6. Commentontherelationshipbetweentheslopesofthethreelinesandthe
numberofionsinsolutionforeachelectrolyte.
7. Commentontheeffectofa)molarconcentrationandb)numberofionsin
solutionontheelectricalconductivityofasolution.