2014 AP' CHEMISTRY FREE.RESPONSEOI'ESTIONS
CIIEMISTRY
Section II
7 Questions
Time-9o minut€s
YOU MAY USE YOUR CALCULATORFOR THIS SECTION.
Directions: Questions l-3 arc long free-responsequestionsthat require about 20 minu@seachto answerand are
worth l0 points each.Questions/t-7 are shon fre€-responsequestionstha! require abou! 7 minutes €achto answer
snd are worth 4 points each.
may be includedin
in the spaceprovidedfollowingeachquestionExamplesandequations
write your response
involvedin ariving
the
steps
the
method
used
and
clearly
show
your responseawhere
For
calculations,
approPriate.
siSnificant
figures'
your
attention
to
fbr
answer'
Pay
your
receive
credit
your
show
work
to
answers,
You
must
it
Massof KI tablet
0.425E
Massof thoroughlydriedfilter pap€r
1.462
E
Massof filter paper+ precipitateafterfirst drying
aftersecond
drying
Massoffilter paper+ precipitate
t.699s
Massof filter paper+ precipitateafterthird drying
g
1.698
I . A studentis giventhe taskof determiningthe I - contentof tabletsthatcontainKl andan inert,water-soluble
is
sugarasa filler. A tabletis dissolvedin 50.0mL ofdistilled water,andan exccssof0.20 M Pb(NO3)2(aq)
addedto the solution, A yellow precipitate forms, which is then filter€d, washed'and dried The data from the
experiment are shown in the table above.
(a) For the chemicalreactionlhat occurswhenthe precipitateforms,
(i) write a balanced,nelionic equationfor the reaction,and
(ii) explainwhy the reactionis bestreFesentedby a net_ionicequation.
(b) Explainthe purposeof dryingandweighingthe filter paperwith the precipitatethr€etimes
(c) In the filtral€ solution,is [K+l greaterthan,lessthan,ot equalto [NO3-] ? Justifyyour answer'
(d) Calculatethe number of moles of p.ecipitate that is pmducedin the exp€riment.
(e) Calculatethe masspercentof I in the tablet
(tl In anotheririal, the stud€ntdissolvesa tablet in 55 0 ril of water insteadof 50 0 ml of watet Pr€dict
whether the experimentally determinedmasspercentof l- will be $eater than, less than. or equal to the
amountcalculatedin part (e). Justify your answer.
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2oI' AP. CHTMIATBY TnIE-RESPOI|S
OIIESTIOIII
(g) A studentin anothc!l6b alsowantsto datlrmin€the I- contantof a Kl tablatbut doesnot haveaccessto
Pb(NOt2. Howcvcr,thestudentdocshaveaccessto 0.20tt AgNO3,whichrcactswith l-(dq) to producc
Agl(s).Thcvalucof ff, for AgI is E.5x l0r!7.
(i) Will thesubstitutionof AgNO3for Pb(NOJ2rtdult in thc prEcipilationol the I- ion ftom solution?
Jultrfy your answcr.
(ii) Thesludcntonly hasacc€3s
!o onc Kl tablctand6 balancrthatcanmalsureto thc ncaEst0.01g.
Will thc .tudcnabc ablcto dct€dnincthc malsof AgI produc.dto thrccsignificantfigurcs?Justify
youl an6wcr.
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201SAp Cheftisw- Lab_Bss€d
euestons
2011 APO CHEMISTRY FREE.RESPONSE OI'ESTIONS
taskof derermining
rhemasspercenr
of sitverin ana oy of copperandsitverby
1"r:*1,_!
orssorvrng
a 114:1:!:
sdnpleot rhealloyin excess
nirricacidandthenprecipiraring
thesiWcraslict.
Firstthestudcntprcpares50.mL of 6 M HNo3,
(a) Thestudentis pfovidedwith a stocksolutionof | 6 M HNo3, rwo 1oomL graduated
cylindersthatcanbe
rcadto ll mL, a 100mL beakerthatcanbereadto tlo mL, safetygoggles,rubbcrgloves, glass
a
stidng rod,a dropper,anddistill€d H2O.
(i) Calculatethevolume,in rnl-, ol 16 ,1,HNO3thatthc studcntshouldusefor preparing
50. mL of
6M HNOI.
(ii) Bri.fly list rhestcpsof anappropriate
andsafeproc.durelor prcparingthe50. mL of 6 M HNO3,Only
matcrialsrclcctedfrom tho6aprovidcdto thc student(list€dabove)maybeused,
(iii) Explainvhy it is nef ncc.ssarym usea volumetricflask(calibrat.dto 50,00
rnl- 10.05nL) to pcrform
Ina d utlon.
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3- A studentis instructedto determinethe concentrationofa solutionofCoCl, basedon absorptionoflight
(sDectrometric/colorimetric
method).Tbe studentis ptovided with a 0 l 0 M and determinesthe percent
the
absorbance
at variouswavelengthsTh€ two graphsbelow representthe data'
transmittanceand
100
a
€ao
g o.4o
-F eo
E
E
\
{ o,zo
4tJ
20
3610 4$t
44A 4&)
.w.v.lcloirr
52()
(nm)
560
dfi)
360
400
it40
4ao
'Wawl.natb
5?o
(aa)
560
6(x)
(a) Whatis theoptimumwavelength
for thisexperiment?
The
theabsorbance
ofthe0.020M 0.040M 0.060M, 0.080M and0 10Msolutions
Thestudent
measures
dataareplotted
below.
o.@
8
q
0.50
0..10
.R0.30
o,m
0.10
0.020
0.040 0.0@ 0,080
Corct ration (It)
0.lo
(b) Theabsorbance
ofthesolution?
is 0 275 whatistheconcentmtion
solution
oftheunknown
whereas
using
ofCoCl2 solutionappropriate.
(c) Why is thismethodofdeterminingtheconcentration
be
appropriate?
would
not
ofNacl
solution
the
concentration
thesamemethodfor measuring
(e.&,testtubeor cuvette)thatholdsth€unknownsolution
container
(d) Thesnrd€nt
thesampLe
handles
of th€
and leavesfing€rprintsin the paih ol the light beam will the calculatedconcentration
your
Explain
or
remain
unchanged?
than
it
should
be,
it
be,
lower
higher
than
should
unknownbe
/ii
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