Level 3 Chemistry (91392) 2017 - NZQA
12
913920 3 SUPERVISOR’S USE ONLY 91392 © New Zealand Qualifications Authority, 2017. All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority. ASSESSOR’S USE ONLY TOTAL Level 3 Chemistry, 2017 91392 Demonstrate understanding of equilibrium principles in aqueous systems 2.00 p.m. Wednesday 15 November 2017 Credits: Five Achievement Achievement with Merit Achievement with Excellence Demonstrate 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.
Transcript of Level 3 Chemistry (91392) 2017 - NZQA
913920
3SUPERVISOR’S USE ONLY
9 1 3 9 2
© New Zealand Qualifications Authority, 2017. All rights reserved.No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
ASSESSOR’S USE ONLY
TOTAL
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.
2
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
(b) 40.0mLof0.150molL–1HBrsolutionwasaddedto25.0mLofasaturatedsilverbromide,AgBr,solution.
(i) WriteanequationfortheequilibriumoccurringinasaturatedsolutionofAgBr.
(ii) ExplainthechangesthatoccurtotheconcentrationsofthespeciesinthesaturatedsolutionofAgBrontheadditionoftheHBrsolution.
(iii) Calculatetheconcentrationofthesilverions,Ag+,aftertheHBrsolutionhasbeenadded.
Ks(AgBr)=5.00×10–13
AssumetheconcentrationofBr–intheoriginalsaturatedsolutionofAgBrisinsignificant.
3
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
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.
4
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
(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.
5
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
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
6
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
(b) (i) Theethanoicacidsolution,CH3COOH(aq),hasapHof2.77beforeanyNaOHisadded.
ShowbycalculationthattheconcentrationoftheCH3COOHsolutionis0.166molL–1.
(ii) CalculatethepHofthesolutionintheflaskafter10.0mLof0.112molL–1NaOHhasbeenaddedto20.0mLofethanoicacidsolution,CH3COOH(aq).
7
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
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.
8
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
9
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.
10
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.
11
Chemistry 91392, 2017
ASSESSOR’S USE ONLY
QUESTION NUMBER
Extra paper if required.Write the question number(s) if applicable.
91
39
2
