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Level 3 Chemistry, 201791392 Demonstrate understanding
of equilibrium principles in aqueous systems
2.00 p.m. Wednesday 15 November 2017 Credits: Five
Achievement Achievement with Merit Achievement with ExcellenceDemonstrate understanding of equilibrium principles in aqueous systems.
Demonstrate in-depth understanding of equilibrium principles in aqueous systems.
Demonstrate comprehensive understanding of equilibrium principles in aqueous systems.
Check that the National Student Number (NSN) on your admission slip is the same as the number at the top of this page.
You should attempt ALL the questions in this booklet.
A periodic table is provided on the Resource Sheet L3–CHEMR.
If you need more room for any answer, use the extra space provided at the back of this booklet and clearly number the question.
Check that this booklet has pages 2 – 11 in the correct order and that none of these pages is blank.
YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION.
QUESTION ONE
(a) Hydrogenfluoride,HF,andhydrogenbromide,HBr,bothformacidicsolutionswhenaddedtowater.
(i) Writeanequationforthereactionofeachacidwithwater.
Hydrogenfluoride,HF,withwater:
Hydrogenbromide,HBr,withwater:
(ii) Compareandcontrasttheelectricalconductivityof0.150molL–1solutionsofhydrofluoricacid,HF,andhydrobromicacid,HBr.
Inyouranswer,youshould:
• includetherequirementsforasolutiontoconductelectricity
• identifythespeciespresentANDtheirrelativeconcentrations.
No calculations are necessary.
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(b) 40.0mLof0.150molL–1HBrsolutionwasaddedto25.0mLofasaturatedsilverbromide,AgBr,solution.
(i) WriteanequationfortheequilibriumoccurringinasaturatedsolutionofAgBr.
(ii) ExplainthechangesthatoccurtotheconcentrationsofthespeciesinthesaturatedsolutionofAgBrontheadditionoftheHBrsolution.
(iii) Calculatetheconcentrationofthesilverions,Ag+,aftertheHBrsolutionhasbeenadded.
Ks(AgBr)=5.00×10–13
AssumetheconcentrationofBr–intheoriginalsaturatedsolutionofAgBrisinsignificant.
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QUESTION TWO
(a) Ammonia,NH3,isaweakbase.
pKa(NH4+)=9.24 Ka(NH4
+)=5.75×10–10
(i) CalculatethepHofa0.105molL–1NH3solution.
(ii) Dilutehydrochloricacid,HCl,isaddedtotheNH3solutionuntiltheratioofNH3toNH4
+inthesolutionis5:1.
DeterminethepHofthissolution,andevaluateitsabilitytoresistachangeinpHwhensmallvolumesofstrongacidorbaseareadded.
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(b) (i) Writetheequationfortheequilibriumoccurringinasaturatedsolutionofcopper(II)hydroxide,Cu(OH)2.
(ii) WritetheexpressionforKs(Cu(OH)2).
(iii) CalculatethesolubilityofCu(OH)2inwaterat25°C.
Ks(Cu(OH)2)=4.80×10–20
(c) ExplainwhythesolubilityofCu(OH)2increaseswhendilutehydrochloricacidisadded.
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QUESTION THREE
Atitrationwascarriedoutbyadding0.112molL–1sodiumhydroxidesolution,NaOH(aq),to20.0mLofethanoicacidsolution,CH3COOH(aq).
Theequationforthereactionis:
CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O() Ka(CH3COOH)=1.74×10–5
Titration curve for CH3COOH versus NaOH
00 5 10 15
Volume of NaOH added (mL)
pH
20 25 30 35 40 45 50 55
2
4
6
8
10
14
12
(a) Withreferencetothetitrationcurveabove,putaticknexttotheindicatormostsuitedtoidentifytheequivalencepoint.
Indicator pKa Tick ONE box below
Methylyellow 3.1
Bromocresolpurple 6.3
Phenolphthalein 9.6
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(b) (i) Theethanoicacidsolution,CH3COOH(aq),hasapHof2.77beforeanyNaOHisadded.
ShowbycalculationthattheconcentrationoftheCH3COOHsolutionis0.166molL–1.
(ii) CalculatethepHofthesolutionintheflaskafter10.0mLof0.112molL–1NaOHhasbeenaddedto20.0mLofethanoicacidsolution,CH3COOH(aq).
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Question Three continues on the following page.
(c) TheequivalencepointpHforthetitrationofethanoicacidwithsodiumhydroxideis8.79.
(i) Identifythechemicalspeciespresentattheequivalencepoint,otherthanwater.
(ii) Inasecondtitration,a0.166molL–1methanoicacidsolution,HCOOH(aq),istitratedwiththeNaOHsolution.TheequivalencepointpHforthistitrationis8.28.
TheequivalencepointpHfortheCH3COOHtitrationis8.79.
CompareandcontrastthepHvaluesattheequivalencepointforbothtitrations.
Ka(HCOOH)=1.82×10–4 Ka(CH3COOH)=1.74×10
–5
No calculations are necessary.
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QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.
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QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.
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QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.