3 percent copper mm - creighton...

Experiment310September2019

PercentCopperandMolarMassofaCopperCompoundSo,let’ssayIstartedwithabout1.0billiongramsofunknownthatisonly40%copper…

...thatwouldbe…400thousandkgofcopper

…at$6/kg…

…or$2.4million.Boom.

Wearegoingtoneedabiggerscale.

Objectives:Todeterminepercentcopperandmolarmassofanunknowncoppersalt.

Overview:1.  Overviewformulas,molarmassand%Cu2.  ProcedureOverview3.  Calculations4.  Procedure5.  Yourlabreport

So,wearemakingcoppertoday?

Well,yeah...butthat’snotthepoint.Wewillfigureouthowmuchcopperisinanunknowncoppersaltandthemolarmassoftheunknown.And

that’sprettyawesome!

2

Remember,beforecomingtolab,youshouldwriteanintroductionthatincludestheobjectivefromthisslideandincludesimportantequationsandconceptsfromthefollowing

slidesthataremarkedwiththissign.

InfoforIntroduction

1.Overview,formulas,molarmass,%Cu

3

Cu2+Cu2+

Cu2+

Cu2+

Cl-

Theunknownmaybeahydrate–asaltthatincludeswaterinitsformula.Ornot.

Forexample,copper(II)

chlorideexistsasanhydrous(nowatersofhydration)orasadihydrate(2watersof

hydration).

Todaywewillanalyzeapuresaltofcopper(II)–maybeit’scopper(II)nitrate,orcopper(II)acetate,orcopper(II)chloride,

andsoon.Wejustdon’tknow.It’sanunknown!

Examplesofsolidcopper(II)chloride:

Asadihydrate:CuCl2.2H2O(s)Orasananydroussolid:CuCl2(s)

Eitherway,insolutionit’sjustions:

Cl-Cl-

Cl-

Cl- Cl-

Cl-Cl-

CuSO4(s)?Cu(NO3)2(s)?

Cu(C2H3O2)2(s)?

4

Continuingwithcopper(II)chlorideasanexample,theanhydrousversionandthedihydrateversionhave

differentmolarmassesandpercentscopper.

Anydrouscopper(II)chloride:CuCl2(s):

MM=1x63.55(Cu)+2x35.45(Cl)=134.45g/mol%Cu=100%x

=47.27%Cu

1x63.55134.45

Asadihydrate:CuCl2.2H2O(s):

MM=1x63.55(Cu)+2x35.45(Cl)+4x1.008(H)+2x15.999(O)=170.48g/mol

%Cu=100%x

=37.28%Cu

1x63.55170.48

1.Overview,formulas,molarmass,%Cu

2.ProcedureOverview(page21)

5

Mg2+

Mg2+Mg2+

Mg2+

X-

Weuseexcessmagnesiumtomakesurewereduceallof

theCu2+iontoCumetal

Westicktothelabmanual’sproceduretoday.Wewilldissolveourunknowncoppersaltinwaterandreducethecopper(II)iontocoppermetalwith

magnesium.Hereisthenetionicequation.ThespectatorionsareunknowntousandareshownasX-inthebeakers.TheymightevenbeX2-.

Cu2+(aq)+Mg(s)àCu(s)+Mg2+(aq)

X-X-

X-

X- X-X-X-

InfoforIntroduction

Cu2+

Cu2+ Cu2+Cu2+

X- X-X-

X-

X- X-X-X-

Mg Mg MgMgMg Cu CuCuCuMg

Stir

Cu2+istheL.R.Justsayin’…

6

Mg2+

Mg2+Mg2+

Mg2+

X- X-X-

X-

X- X-X-X-

InfoforIntroduction

Cu2+

Cu2+ Cu2+Cu2+

X- X-X-

X-

X- X-X-X-

Mg Mg MgMgMg Cu CuCuCuMg

Stir

Cu2+(aq)+Mg(s)àCu(s)+Mg2+(aq)

Youcantellwhenthereactionisover–alloftheblueCu2+isgoneandthesolutioniscolorless.Youshouldseeorangecoppermetal.

NextwehavetogetridofexcesssolidMgoritwilladdtotheweightofthesolidcopperandmessup

ourresults.


7

Mg2+

Mg2+Mg2+

Mg2+

X- X-X-

X-

X- X-X-X-

InfoforIntroduction

Cu CuCuCuMg

H3O+

AcidoxidizesMgtoaqueousMg2+.Weaddacidandstiruntilthebubblesstop.

Mg2+

Mg2+Mg2+

Mg2+

X- X-X-

X-

X-X-X-X-

Cu CuCuCu

Mg(s)+H3O+(aq)àMg2+(aq)+H2(g)+H2O(l)

Mg2+

Afterthesolidmagnesiumisgone(nomorebubbles),wecanfilterthesolutionandcollectthecoppermetal.

Stir


8

2.Procedurefromlabmanual

Nowit’stimeforyoutodotheexperiment Followingthe

procedureinthelabmanual

Whenyouaredonewiththeprocedure,comebacktothe

nextslide

…forthecalculations!

3.Calculations

9

Fromthemassofcopperwecangetmolesofcopperandbecausetheunknownhasonlyonecopperatom,themoles

ofunknownequalsthemolesofcopper!Easypeasy.

%Cu=100%x

mCunCununknownPercentcopperandmolarmassareeasy

formulastouse.Watchoutforsignificantfiguresandunits.

MM=mCu

munknown

munknown

nunknownInfofor

calculations

10

Soweendupwithtwoimportantmeasurementstoday–themassoftheunknownandthemassofcopperintheunknown.

%Cu=100%x

Gettingthemassofcopperrightisthetrick.Ifyoulose

someCuduringfiltration,your%Cuwillbetoolow.IftheCuiswet,themasswillbetoohighandsowillthe%Cu

mCu

munknown

Infoforcalculations

Notsoeasytomessupthemassofthe

unknown…

3.Calculations

11

Soifourmassofcopperistoolow,ourpercentCuwillalsobetoolow.Butwhataboutthemolarmass?

InfoforSourcesofError

mCunCununknown

Rememberhowmassofcoppergivesusmolesofcopperandthatgivesus

molesofunknown?Yeah.Soerrorswiththemassof

copperwillcauseerrorswiththemolarmassaswell.

MM=munknownnunknown

3.Calculations

%Cu=100%xmCu

munknown

SO4

Hmmm...IfIgetthemassofthecopperright,Ishouldbegood.

Thisisthefirstexperimentinwhichwearebeinggradedfor

accuracy!

…andsigfigs

…andunits

3.Calculations

12Yourcopperisworthabout0.72cents

SO4

13

Herearesomecompoundsofcopper.Ourunknownis

oneofthese.

Infoforcalculations

CuBr2 MM=223.4g/molCu(C2H3O2)2.H2O MM=199.6g/molCuCl2.2H2O MM=170.5g/molCuCO3 MM=123.6g/molCuSO4

.5H2O MM=249.7g/mol

Whichoneisit?Andwhatisthepercenterrorformolar

mass?

%Error=100%xdifferenceactual

Sometimesyoucan’tdecidebetweentwo.Concludeitmaybeeither–don’tjustpickone–that’snotgoodscience.BTW,Ihavealuncheonrightafterthis.

Hencethetux.

3.Calculations

4.Procedurefortoday(pg.21)

14

I.  Wearyoursafetyglassestoday.Andweneedtodressforamesstoday.

II.  Wefollowthemanualcarefully.

III.  Useananalyticalbalanceformeasuringmassesofcopperunknownandcoppertoday.Usethelittlescaleatyourstationtomeasurethemagnesium.

IV.  Recordobservationsanddetailsascarefullyaspossible.

V.  Conclusions.We’lllearnmoreonthenextslide.

VI.  InsteadofStep11,turninyourcopperinaweighingdish.Makealabelwithyournamesandlabstationandsection.

Intheconclusionwecansummarizewhatwe’velearned.Whydidwedothisexperiment?ReviewtheObjectivesfromSlide2andseeifwedidwhatwesetouttodo.Wereadyourconclusionscarefully.Besuretowriteitinyourownwordsandnotcopyitfromanyone.

15

Conclusion.In this experiment we determined the percent copper in an unknown salt

of copper(II). To do this we needed the mass of the unknown salt and the mass of the copper it contained. In our case, we used 2.514 g unknown and ended up with 1.250 g copper metal, which is 0.1967 mol Cu. That works out to 49.72% copper using the formula %Cu = 100% X mass Cu/mass unknown.

We also determined the molar mass of the unknown using the formula MM = mass unknown/moles unknown. The moles of unknown is the same as the moles of copper. We got 2.514 g/0.01967 moles = 127.8 g/mol, which is most similar to CuCO3 from the list of choices. If that is actually the identity of the unknown, we are off by just 3.4%. None of the other choices were even close, so we are confident about the identity of our unknown.

5.Yourlabreport.

Wealsoaddresssourcesoferrors.Sometimestherearen’tanyobviousonesandothertimesthereareplenty.Weneedtoonlyworryaboutthemoreplausibleones.Lookingoverthecontinuumofpossibleerrorsfor

thisexperiment,wewillstickwiththereasonableonesonly.

Reasonable UnreasonableNottoolikelyCopperwaswetor

SomecopperwaslostUsedthebalanceincorrectly

SamplewasimpureLawsofphysicssuspended.

SabotagedbyTAs

5.Yourlabreport.

16

Sources of error.In the context of our good results, errors probably were not

significant for us. However, we can see how maybe some copper could have been lost during the filtering. If that had happened, the percent copper would be too low and the molar mass too high. On the other hand, if the copper were a bit wet or had some magnesium in it, the percent copper would calculate to be too high and the molar mass too low. We feel that no major errors were encountered.

5.Yourlabreport:EnteringOn-lineresults.

Todayisthefirstexperimentinwhichweenterourresultson-line.ClickthelinkattheChm204websitewhereitsaysSubmiton-lineresults(forExperiment3).Besupercarefultonotmakecareless

mistakes.Thevaluesyouenterwillbeusedtocheckyourcalculations.Thestuffyouenterisworth15pointsthisweek.Youandyourlabpartnernormallyentervaluestogether.Referbackto

theorientationpresentationifyouhaveanyquestions.

NumbersmustbeenteredinMicrosoft’sExcelformatonlyifyouareusing

exponentials.Thatformatlookslike1.234E-5or6.789E10.Thereareno

spacesoneithersideoftheE.Numberscanalsobeenteredasdecimalswithouttheexponentialnotation–like0.001234,

forexample.

Don’tmessup.

17

5.Yourlabreport:EnteringOn-lineresults.

18

CanIenter1.234x10^5?

Howabout1.234E^5?

Isthisok:1.234E5?

Whatabout…

No.

No.

No.

No.

Only1.234E5works.That’sthewayitis.

5.Yourlabreport.

①  First,thecoverpagewithTAinitials.②  Next,thetrimmedcopypagesfromyourlabnotebook

stapledtogether.③  Enteron-linedatabeforeyouleavelab.Yourcalculations

willbecheckedaswellascorrectuseofunitsandsignificantfigures.Latesubmissionsarenotgraded–seethesyllabus.

④  Turnedinlabreporttodayorbeforethestartofclasstomorrow.Latelabsmaynotbegraded–seethesyllabus.

StickpeopleinspiredbyxkcdcartoonsbyRandallMunroe 19

Ourworkhereisdone

Let’sgo!

ChemLabwiththeStickPeopleandBirdwascreatedandproducedbyDr.BruceMattson,CreightonChemistry.Enjoyitandshareitifyouwish.

3 percent copper mm - creighton...

Documents