dse chem pp ans

8/10/2019 DSE CHEM PP Ans

1/23

HONG KONG EXAMINATIONS AND ASSESSMENT AUTHORITY

HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION

PRACTICE PAPER

CHEMISTRY PAPER 1

MARKING SCHEME

(2012222)

(updated as at 22 Feb 2012)

This marking scheme has been prepared by the Hong Kong Examinations and

Assessment Authority for teachers and students reference. This markingscheme should NOT be regarded as a set of model answers. Our examinationsemphasise the testing of understanding, the practical application of knowledgeand the use of processing skills. Hence the use of model answers, or anythingelse which encourages rote memorisation, will not help students to improve theirlearning nor develop their abilities in addressing and solving problems.

Hong Kong Examinations and Assessment AuthorityAll Rights Reserved 2012

PP-DSE-CHEM 11


2/23

Part I

Question No.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.22.

23.

24.

Part II

Question No.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

Key

B

D

A

D

C

B

B

C

B

B

B

A

A

C

D

A

D

A

C

C

BA

D

D

Key

A

D

A

B

C

A

D

B

D

C

B

C

SECTION A

PP-DSE-CHEM12


3/23

SECTION B

General Notes for Teachers on Marking

1. Themarkingschemehasbeenupdated,withrevisionsmadeafterthescrutinyofactualsamplesofstudentperformance in the practice papers. Teachers are strongly advised to conduct their own internal

standardisation procedures before applying the marking schemes. After standardisation, teachers shouldadheretothemarkingschemetoensureauniformstandardofmarkingwithintheschool.

2. Themarkingschememaynotexhaustallpossibleanswersforeachquestion. Teachersshouldexercisetheir

professionaldiscretionandjudgmentinacceptingalternativeanswersthatarenotinthemarkingschemebut

arecorrectandwellreasoned.

3. Thefollowingsymbolsareused:

/ Asingleslashindicatesanacceptablealternativewithinananswer.

* Step-mark(forquestionsinvolvingcalculations)

Correctspellingrequired

4. Inquestionsaskingfora specified number ofreasonsorexamplesetc.anda studentgivesmore thanthe

requirednumber, the extraanswers should not bemarked. For instance, ina questionaskingstudentsto

providetwoexamples,andifastudentgivesthreeanswers,onlythefirsttwoshouldbemarked.

5. Forquestionsinvolvingcalculations,thenumberofsignificantfiguresincandidatesfinalanswersshould

tallywiththatgiveninthequestion.

6. Chemical equations should be balanced except those in reaction schemes for organic synthesis. For

energetics, thechemicalequations givenshouldincludethe correct statesymbolsof thechemicalspecies

involved.

7. Inthequestionpaper,questionswhichassesscandidatescommunicationskillsaremarkedwithanasterisk

(*).Forthesequestions, the mark for effective communication (1markper question) will beawardedif

candidatescanproduceparagraph-lengthanswerswhichareeasilyunderstandable. Nomarksforeffectivecommunication will be awarded if the answers produced bycandidates are written innote form, and/or

containalotofirrelevantmaterials.

PP-DSE-CHEM13


4/23

Part I

Marks

1. (a) (i) ZnO+H2SO4 ZnSO4+H2O 1

or, ZnO+2H+ Zn2++H2O (1)

(ii) UnreactedZnO(s)canbeseen. 1

(iii) Toensurethattheproductisnotcontaminatedwithsulphuricacid. 1

or, Theunreacted ZnO(s) canberemoved by filtration,but it is difficulttoremovethe (1)

excessH2SO4(aq).

(3)

(b) Removeadropofthesolutionwithaglassrod,andseewhetheranysolidformswhenthe 1

dropcools.

(Acceptothercorrectanswers.)

(1)

(c) Washingwithdistilledwatercanremovethewater-solubleimpurities. 1

Usingasmallamountofwater/coldwaterhelpsreducelossofthesalt. 1

(2)

(d) AnyONEofthefollowing: 1

dryingthecrystalsbetweenfilterpapers (1)

puttingthecrystalsinadesiccator. (1)

(DONOTacceptmethodswhichinvolvestrongheating.)

(1)

(e) Zn/Zn(OH)2/ZnCO3 1

(1)

PP-DSE-CHEM14


5/23

Marks

2. (a)

or,

Somecomponentsofwine(substanceswithapleasantodour)canbeoxidisedbyoxygenin

airtogiveproductsthathaveaflattaste.

Ethanolinwinecanbeoxidisedbyoxygeninairtogiveethanal/ethanoicacid.

1

(1)

(1)

(b) (i) Theoutermostshellofanargonatomisastableoctetstructure.

formbondswithotheratoms.

Ardoesnotreadily 1

(ii) Arisdenserthanair.

contactwithair.

Itdisplacesairfromthebottle,andthuspreventsthewinefrom 1

(iii) Heislessdensethanair. Itwillnotdisplaceair/itwilleasilydiffusefromthebottle. 1

(3)

(c) Thesubstanceswithapleasantodourarevolatileorganiccompounds.

thebottlemayalsoremovethesesubstances.

Pumpingairoutfrom 1

(1)

PP-DSE-CHEM15


6/23

Marks

3. (a) (i)

1

or,

(1)

(ii) Mg3N2+6H2O3Mg(OH)2+2NH3 1

No.Thereisnochangeinoxidationnumberofanyatom. 1

(3)

(b) (i)

1

(ii) ThenitrogeninNCl3andthatinNH3bothhavethesamenumberofelectronbond-pairs 1andloneelectronpairs/havethreeelectronbond-pairsandoneloneelectronpairin

theiroutermostshells.

The repulsion between these electron pairs causes both NCl3 and NH3 to adopt a 1trigonalpyramidalshape.

(3)

PP-DSE-CHEM16


7/23

Marks

4. (a) Dissolve1.14gofM2CO3(s)insomedistilledwater/deionisedwaterinabeaker. Transfer 1

thesolutiontoa100.0cm3

volumetricflask.

Wash thebeaker with distilled water /deionisedwaterandtransfer thewashings into the 1

volumetricflask.Add distilled water / deionised water up to thegraduationmarkof the volumetric flask. 1

Shakethevolumetricflasktoensureitscontentiswellmixed.(3)

(b) No.ofmolesofH+(aq)used

=0.08525.30103

3 1=2.1510

M2CO3+2H+

2M+

+CO2+H2O

No.ofmolesofM2CO3in100cm3ofthesolution

1*100 13=2.1510

10 2

1.14

2FormulamassofM2CO3 =0.0215

=106 1

Letx betherelativeatomicmassofM

2x +12+16 3=106

x =23

M islikelytobeNa. 1

(4)

PP-DSE-CHEM17

*stepmark


8/23

Marks

5. (a) C H

%bymass 81.8 18.2

81.8 18.2Atomratio : 1*

12 1= 6.82 : 18.2

= 3 : 8

AlkanehasthegeneralformulaCnH2n+2 1

X ispropane/C3H8 . 1

(3)

(b) Fractionaldistillationofthepetroleumgaseousfraction. 1

or, Cracking of naphtha / heavy oil (or any appropriate petroleum fraction)followed by (1)

fractionaldistillationoftheproducts.

(1)

(c) (i) X:C3H8 easilyundergoescompletecombustiontogiveCO2 andH2O. The products 1

poselittleharmtotheenvironment.

(ii) Kerosene:keroseneundergoesincomplete combustiontogive aluminousflame.The 1flamecanbemoreeasilyseen.

(Acceptotherreasonableanswers.)

(2)

*stepmark

PP-DSE-CHEM18


9/23

Marks

6. (a) propane-1,3-diol/1,3-propanediol 1

(1)

(b) Allthreecompoundshaveahydroxylgroup/aremonohydricalcohols. Theboilingpointof 1

thesecompoundsdependsonthestrengthofvanderWaalsforcesbetweenmolecules.

Thestrengthofvander Waalsforces in alcoholsincreases with the carbon chainlength / 1molecularsize. Boilingpointincreasesintheorder:A


10/23

Marks

7. (a) (i) 3.0No.ofmolesofCaO(s)used =

(40.1+16)

=0.053 1*

Heatliberated=53

4.2

(46.728.2) 1*=4118J

4118 H= 1*

0.053

=77.0kJmol1

1

(Acceptablerange:72.6to77.0kJmol1)

(ii) AnyONEofthefollowing: 1

PPisnotaperfectheatinsulator;heatislosttothesurroundings.

SomeCaO(s)mayhavereactedwithH2O(l)inair.


(5)

(b) (i) AnyTHREEofthefollowing(atleast1markshouldbeallocatedtoeachpart): 3

(I) PP is apoorconductor of heat. UsingPPcontainertohold CaO(s) willprotect (1)

handsfromskinburns.

PP can withstand the high temperature caused by the reaction of CaO(s) with(1)

H2O(l).

(II) CompoundsofAlarenon-toxic. Theywillnotcausefoodpoisoning. (1)

Alisagoodconductorofheat. TheheatliberatedfromthereactionofCaO(s)with (1)

H2O(l)canreadilybetransmittedtothecoffeebeverage. AluminiumiscoveredbyalayerofunreactiveAl2O3(s),whichpreventsthemetal (1)

fromcorrosion.


(ii) ThereactionofCaO(s)andH2O(l)ishighlyexothermic,andCaO(s)isaninexpensive 1material. (Acceptotherreasonableanswers.)

(4)

*stepmark

PP-DSE-CHEM110


11/23

Marks

8. (a) anode:CH3OH(aq)+H2O(l) CO2(g)+6H+(aq)+6e 1

cathode:O2(g)+4H+(aq)+4e

2H2O(l) 1

(2)

(b) (i) Methanol does not conductelectricity. It is not suitable to be used asthereaction 1

mediumfortheelectrochemicalreaction.

or, H2Oisinvolvedinthehalf-equations. (1)

or, Acidisinvolvedintheelectrochemicalreaction. (1)

(ii) Toxicandflammable 1

(2)

(c) AcceptbothYesandNoanswers. Markswillbeawardedonlytotheexplanation. 2

ForNoanswers,

Electricalsocketscanbefoundinmostplaces. DMFClaptopcomputerswillonlybe (1)

usedinplaceswhereelectricsocketsarenotavailable. The cost for the manufacture of methanol is higher than that for the generation of

(1)

electricityinmostplaces.

ForYesanswers, The use of DMFC laptop computers will become prevalent when stringent (1)

environmental laws are enforced as the disposal of DMFCs causes less harm to the

environment than other rechargeable cells / methanol is a greener fuel than

hydrocarbons.

DMFClaptopcomputerswillbecommonlyusedinfieldworkwhereelectricsocketsare (1)

notavailable.


(2)

PP-DSE-CHEM111


12/23

Part II

Marks

9. (a)

MgO Al2O3 SiO2 P4O10 SO2

Structure IC IC CN SM SM

Acid-baseproperty

BA AM AC AC AC

1

1

(2)

(b) Ionicoxidesarebasic,whilecovalentoxidesareacidic. 1

(1)

(c) (Inthisquestion,award1markfor thereagentsused ineachoftestsforacidic,basicand 4

amphotericoxides,and1 markfor a correctobservation. One possiblemethod isshown

below.)

AddeachoxidetoHCl(aq)andmeasurethepHofthemixture. OnlyMgO(s)andAl2O3(s)

reactwithHCl(aq)andthepHincreases. Thesetwooxidesdemonstratebasicproperties.AddeachoxidetoNaOH(aq)andmeasurethepHofthemixture. OnlyAl2O3(s),SiO2(s),

P4O10(s)andSO2(g)reactwithNaOH(aq)(SiO 2(s)reactswithhotconc.NaOH(aq)),andthe

pHdecreases. Theseoxidesdemonstrateacidicproperties.

Al2O3(s)reactsinbothcases. Itisamphoteric.

(Acceptotherexperimentalmethod.)

Effectivecommunication(Award1markifcandidatescanexpresstheirideasclearly.) 1

(5)

Marks

10. (a) Colorimetry/usingcolorimeter 1

(1)

(b) (i) TherateofconsumptionofMnO4(aq)ionsisslowatthebeginning(from0to180s) 1

andthenincreasesrapidly(from200to340s).

It is likely to be due to the building up of the concentration of the product which 1catalysesthereaction.

(ii) Repeat the experiment with a few drops of Mn2+

(aq) firstly added to the reaction 1

mixture.ConsumptionofMnO4

(aq)ionswillbefasteratthebeginningifMn2+(aq)isacatalyst. 1

(4)

PP-DSE-CHEM112


13/23

Marks

11.

a: (1)LiAlH4/ether;(2)H3O+ 1

b:

1

c: excessconc.H3PO4(orconc.H2SO4);heat/Al2O3(s);heat

(3)

Marks

12.(a) (i)

1+1

(ii) They turn the plane of polarisation of a beam of plane polarised light in opposite1

directions.

or, Oneofthecompoundsislaevorotatorywhiletheotherisdextrorotatory. (1)

or, Crystalsofthetwocompoundshavedifferentappearance. (1)

(3)

(b) Repeatingunit: 1

(1)

PP-DSE-CHEM113

1


14/23

Marks

13.(a)

cK

43

2

2

43

(aq)][NH (aq)][Cu

(aq)][Cu(NH )+

+

= 1

(1)

(b)4(0.002)(0.0014)

0.08=

cK

=1.041013(moldm3)4

(1markforanswer;1markforcorrectunits)

1+1

(2)

(c)

or,

H2SO4(aq)reactswiththeNH3(aq)present:

H+(aq)+NH3(aq) NH4

+(aq)

H2SO4(aq)+2NH3(aq) (NH4)2SO4(aq)RemovalofNH3(aq)causesthepositionofthefollowingequilibriumtoshifttotheleft.

Cu2+(aq)+4NH3(aq) Cu(NH3)42+(aq)

NH3(aq)isaweakbase:

NH3(aq)+H2O(l) NH4+(aq)+OH(aq)

When[Cu2+(aq)]buildsup,itwillreactwiththeOH(aq)ionstogivetheblueprecipitate.

Cu2+

(aq)+2OH(aq) Cu(OH)2(s)

WhenexcessH2SO4(aq)is added,itwillreact withtheCu(OH)2(s)formedtogiveablue

solution.

Cu(OH)2(s)+2H+(aq) Cu

2+(aq)+2H2O(l)

(3marksforchemicalequations;1markforexplanationoftheshiftinequilibriumposition;

1markfortheformationofblueprecipitate.)

1

(1)

1

1

1

1

(5)

PP-DSE-CHEM114


15/23

HONG KONG EXAMINATIONS AND ASSESSMENT AUTHORITY

HONG KONG DIPLOMA OF SECONDARY EDUCATION EXAMINATION

PRACTICE PAPER

CHEMISTRY PAPER 2

MARKING SCHEME

(2012222)

(updated as at 22 Feb 2012)

This marking scheme has been prepared by the Hong Kong Examinations and

Assessment Authority for teachers and students reference. This markingscheme should NOT be regarded as a set of model answers. Our examinationsemphasise the testing of understanding, the practical application of knowledgeand the use of processing skills. Hence the use of model answers, or anythingelse which encourages rote memorisation, will not help students to improve theirlearning nor develop their abilities in addressing and solving problems.

Hong Kong Examinations and Assessment AuthorityAll Rights Reserved 2012

PP-DSE-CHEM 21


16/23

General Notes for Teachers on Marking

1. Themarkingschemehasbeenupdated,withrevisionsmadeafterthescrutinyofactualsamplesofstudent

performance in the practice papers. Teachers are strongly advised to conduct their own internalstandardisation procedures before applying the marking schemes. After standardisation, teachers should

adheretothemarkingschemetoensureauniformstandardofmarkingwithintheschool.

2. Themarkingschememaynotexhaustallpossibleanswersforeachquestion. Teachersshouldexercisetheir

professionaldiscretionandjudgmentinacceptingalternativeanswersthatarenotinthemarkingschemebut

arecorrectandwellreasoned.

3. Thefollowingsymbolsareused:

/ Asingleslashindicatesanacceptablealternativewithinananswer.

* Step-mark(forquestionsinvolvingcalculations)

Correctspellingrequired

4. Inquestionsaskingfora specified number ofreasonsorexamplesetc.anda studentgivesmore thanthe

requirednumber, the extraanswers should not bemarked. For instance, ina questionaskingstudentsto

providetwoexamples,andifastudentgivesthreeanswers,onlythefirsttwoshouldbemarked.

5. Forquestions involvingcalculations,thenumberofsignificantfigures incandidatesfinal answersshould

tallywiththatgiveninthequestion.

6. Chemical equations should be balanced except those in reaction schemes for organic synthesis. For

energetics, thechemicalequations givenshouldincludethe correct statesymbolsof thechemicalspecies

involved.

PP-DSE-CHEM22


17/23

1. (a) (i)

(ii)

(iii)

(iv)

(b) (i)

(ii)

Marks

AnyONEofthefollowing: 1

ReichsteinprocessisusedforsynthesisingvitaminC,whichisingreatdemandasit (1)

cannotbesynthesisedinhumanbody.

ReichsteinprocessprovidesasyntheticroutetoconvertD-Glucose,asugarwhichis (1)highlyabundantandcheap,toL-ascorbicacid.

The enzymatic reaction in Reichstein process inverts the sugars from D- to L- (1)isomer.


(1)

reduction/catalytichydrogenation 1

(1)

(I) CommonlyusedoxidisingagentswillalsooxidisetheotherhydroxylgroupsinD- 1

sorbitol/arenotselectiveascomparedwiththeenzyme.

or, TheenzymecanselectivelyoxidisethesecondOHgroupinD-sorbitoltogiveL- (1)

sorbose.

(II) AtpH6,theenzymewillundergodenaturation/the(tertiary/secondary) 1

structureoftheenzymewillchangeleadingtolossofcatalyticactivity.

(2)

Thismethodusesacatalyst(theenzyme)insteadofstoichiometricreagents. 1

Ituseslesshazardouschemicals(e.g.MeOH/acid). 1

(2)

AnyONEofthefollowing: 1

Therateofreactionisproportionaltotheconcentrationofthereactant. (1)

Thehalf-lifeofthereactionisconstant. (1)

(1)

Ealogk =constant 12.3RT

T 313 323 333 343

k/10-3 27.0 35.4 50.4 65.4

1/T 3.195103

3.096103

3.003103

2.915103

logk 1.569 1.451 1.298 1.184

correctspelling

PP-DSE-CHEM23


18/23

2

Marks

1. (b) (ii)

1Graphoflogk against

T

(1markforthegraph;1markforthelabels)

R

Ea

3.2

14028.312.3

RT

1/T/K1

Slope:1402=

Ea =

= 26.8kJmol1

(Acceptablerange:24.5to29.0kJmol1)

E=constant a

1

1

(1)or,

lnk

T 313 323 333 343

k/103

27.0 35.4 50.4 65.4

1/T 3.195

103

3.096

103

3.003

103

2.915

103

lnk 3.612 3.341 2.988 2.727

1Graphoflnk against (2)T

1/T/K1

ln

k

(1markforthegraph;1markforthelabels)

EaSlope:3227= R

32278.31

26.8kJmol1

(Acceptablerange:24.5to29.0kJmol1

)

(1)

Ea =

= (1)

(5)

PP-DSE-CHEM24


19/23

Marks

1. (c) (i) At the cathode, H+(aq) is preferentially discharged because H

+occupies a lower

positionthanNa+

intheelectrochemicalseries.

2H+(aq) + 2e

H2(g)

Attheanode,Cl(aq)ispreferentiallydischargedbecauseitsconcentrationishigh.

2Cl(aq) Cl2(g) + 2e

Themembraneonlyallowscationstopassthroughit.

Asthereisahigherconcentrationofcationsintheanodecompartment(becauseanions

areremoved),therewillbeanetflowofNa+(aq)ionsfromtheanodecompartmentto

thecathodecompartment,andconcentratedNaOH(aq)isformed.

1

1

1

1

1

(5)

(ii)

or,

Cl2(g)

Cl2(g)

+ 2NaOH(aq) NaOCl(aq) + NaCl(aq) +H2O(l)

+ 2OH(aq) OCl

(aq) + Cl

(aq) +H2O(l)

1

(1)

(1)

(iii) AcceptbothYesandNoanswers.

ForNoanswers,

TheproductionofH2 inchloroalkaliindustryrequirestheuseofelectricity,whichis

commonlygeneratedbyburningfossilfuel(amajorsourceofairpollutants).

Cl2(g) isthemainproductofthechloroalkaliindustry. Largescaleproductionof

H2(g)willyieldsurplusCl 2(g). ThedisposaloftheunusedCl2(g)iscostlyandwillcauseairpollutionproblems.

1

1

ForYesanswers,

BurningofH2(g)giveswateronly.

Ifthereisacheapsourceofelectricity(e.g.photovoltaiccell),productionofH2(g)byelectrolysisofbrinecanbeameansofreducingairpollutionproblems.

(1)

(1)

(Acceptotherreasonableanswers.)(2)

PP-DSE-CHEM25


20/23

Marks

2. (a) (i) (I) face-centredcubic 1

(II) 1 1 1*No. of atoms =8( )+6( )

8 2

= 4 1

(III) 427.0Density=

8 3 23 1*(4.0510 ) 6.0210

=2.7gcm3

1

(5)

(ii) (I) Inaluminiumalloys,theintroductionofatomsofotherelementsintothelattice 1

causesdistortionoftheregulararrangementoftheAlatoms.

Relativemotionbetweenlayersofatomswillbehindered. 1

(II) Liistheleastdensemetal. Li-Alalloyshaveverylowdensitiesandaresuitable 1

materialsformakingaircraftbodies.

(3)

(iii) (I) Biotite has a layered structure. The attraction force between layers is much 1

weakerthantheattractionforcebetweenatomswithinalayer.

(II) asinsulatorincapacitors 1

(2)

(b) (i) (I) 1

(II)

(ii) (I)

(II)

correctspelling

*stepmark

injectionmoulding/vacuumforming 1

(2)

SAN,ingeneral,isnotmadefromstyreneandacrylonitrilein1:1moleratio. 1

Thetwotypesofmonomersdistributerandomlyalongthepolymermolecule. 1

InPS,thepolymermoleculesareattractedtoeachotherbyweakvanderWaals 1

forces.

AcrylonitrilehasapolarCNgroup. 1

TheCNgroupsholdthepolymermoleculestogetherbystrongerdipole-dipole 1

interaction.

(5)

PP-DSE-CHEM26


21/23

Marks

2. (b) (iii) Thecopolymerisathermosettingplasticwiththefollowingstructure: 1

1

.

.

.

(Acceptothercorrectrepresentationsoftheabovestructure.)

Thecross-linkagesmakethecopolymerhardand rigid. Asthecopolymerhasagiant

covalentnetworkstructure,itdoesnotmeltuponheating.

1

(3)

PP-DSE-CHEM27


22/23

Marks

3. (a) (i) carboxylicacid/carboxylgroup(becauseX issolubleinNaOH(aq))

(Alsoacceptphenol.)

1

(1)

(ii) separatingfunnel 1

(1)

(iii)

or,

Step 1 : AllowX todissolveinNaOH(aq)togive(carboxylate)anions/asalt.

Step 2 : Allowthenon-polarimpuritiestodissolveinhexanewhilethe(carboxylate)anions/salttostayintheaqueouslayer.

Separatethenon-polarimpurities(inhexane)fromthesaltofX.

Step 3 : Regeneratethe(carboxylic)acid(whichisinsolubleinwater)byaddingacid.

1

1

(1)

1

(3)

(iv) X possessesaC=CbondbecauseitdecolourisesBr2 inCH3Cl3.

Inthemassspectrum,thepeakatm/z =148isduetothemolecularion.

AnyTWOofthefollowing:

Thepeakatm/z =131isduetothecationformedfromthemolecularionbystripping

offaOH.

Thepeakatm/z =103isduetothecationformedfromthemolecularionbystripping

offaCO2H.

Thepeakatm/z =77showsthatX carriesabenzenering(m/z forC6H5+

ion=77).

1

1

2

(1)

(1)

(1)

PossiblestructuresofX: 1

CO2H

CO2HCO2H

(5)

(v) (I) Putthechromatographicplateintoajarthatissaturatedwithiodinevapour.

spotswillappearbrown.

The 1

or, Irradiate theplatewith UV. Thestationaryphase is fluorescent while the two

spotsarenot.

(Acceptothercorrectanswers.)

(1)

(II)fR

distancetravelledbysolvent

distancetravelledbyX=

2.5)3(509.5

=

44.5

9.5=

=0.21 1

(III) columnchromatography(usingthesamemovingphaseandstationaryphase) 1

(3)

correctspelling

PP-DSE-CHEM28


23/23

Marks

3. (b) (i) Washtheprecipitatethoroughlywithdeionisedwater. 1

Drytheprecipitateinanoven/ensurethattheprecipitateisdriedbeforeitisweighed. 1

(2)

(ii) Ba2+(aq)+SO42(aq)

BaSO4(s)

2+ 0.291 No.ofmolesofBa present= 1*(137.3+32.1+164)

3=1.2510

3MassofBainthesample=1.2510 137.3

=0.171 1*

0.171%bymassofBainthesample=

0.305

=56.1 1

(Acceptablerange:56.0to56.4)

(3)

(iii) AnyTWOofthefollowing: 2

The reaction must be significantly complete, i.e. the precipitate is practically (1)

insoluble. Theproduct(precipitate)shouldhavedefinitechemicalcomposition. (1)

Therateofreactionmustbefastenoughtobepractical. (1)

(Acceptothercorrectanswers.) (2)

*stepmark

dse chem pp ans

Documents