Stage 2 Chemistry
Assessment Type 2: Skills and Applications Tasks
Analytical Techniques Test
Student 1 Response
Credit will be given in Questions 1-3 for the correct use of significant figures.
(1 mark)
Question 1
An experiment was carried out to determine the percentage of calcium carbonate in eggshells.
Step 1: A mass of 0.620 g eggshells was crushed and added to 25.00 mL of 0.300 mol L 1 HCl
and allowed to react until there until no more bubbles were evolved. Some HCl did not
react and remained in the reaction mixture.
CaCO3 + 2 HCl  CaCl2 + CO2 + H2O
Step 2: The HCl which did not react was titrated with a standardised solution of NaOH of
concentration 0.200 mol L1.
NaOH + HCl  NaCl + H2O
A volume of 18.20 mL NaOH was required to neutralise the unreacted HCl.
(a) Name the glassware used to deliver the NaOH and state and explain two steps which must
be followed in order to ensure that the number of moles of NaOH delivered is accurate.
Name:
Burette
Step 1: Must
be rinsed with NaOH solution to avoid dilution of the solution.
Step 2: Must
check that there are no bubbles.
(5 marks)
(b) Calculate the number of moles of HCl initially added to the crushed eggshells.
nHCl initially added = C.V
= 0.300 x 0.0250
= 0.00750 mol (3 sf)
(2 marks)
(c) Calculate the number of moles of NaOH required to neutralise the HCl in Step 2.
nNaOH = 0.200 x 0.0182
= 0.00364 mol (3 sf)
(2 marks)
(d) Hence calculate the number of moles of HCl that remained unreacted at the end of Step 1.
nHCl unreacted = nNaOH = 0.00364 mol (3 sf)
(1 mark)
(e) Hence calculate the number of moles of HCl that reacted with the eggshells.
nHCl reacted = nHCl initially added  nHCl unreacted
= 0.00750  0.00364
= 0.00386 (3 sf)
Page 1 of 10
Application
Over the task,
apart from one
arithmetic error,
correct solutions
to simple and
complex
(1 mark)
quantitative
problems have
been derived.
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
(f) Calculate the percentage, by mass, of calcium carbonate in the eggshells.
nCaCO3 = ½ nHCl
mCaCO3 = n.M
= ½ x 0.00386 x 84.09
% CaCO3 =
½ x 0.00386 x 84.09
0.62
0
Application
Error in calculation of M.
Method and subsequent
calculation are correct.
x 100 = 26.2% (3sf)
(3 marks)
(g) It was suspected that the HCl used in the titration had been contaminated with traces of KOH.
(i)
State whether this is a source of random or of systematic error.
Systematic
(1 mark)
(ii) Explain the effect that this contamination would have had on the titre value.
Titre would be lower because some of the HCl would already have reacted with
the KOH.
Application
Well- reasoned
solution to problem.
(2 marks)
TOTAL: 18 marks
Question 2
Potassium hydrogen phthalate (KHP) is used as a primary standard in acid-base titrations. The
formula of KHP is C8H5O4K.
COO-K+
Potassium hydrogen phthalate
COOH
(a) A standard solution of KHP of concentration 0.100 mol L1 was prepared.
Calculate the mass of potassium hydrogen phthalate needed to prepare 50.0 mL of a
0.100 mol L1 solution.
n = C.V
= 0.100 x 0.0500
m=nxM
= 0.100 x 0.0500 x 204.22
= 1.02 g
(3 marks)
Page 2 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
(b) This 0.100 mol L1 solution was then diluted to make 500.0 mL of solution of concentration
0.00500 mol L1.
(i) Calculate the volume of original solution required to prepare 500.0 mL of diluted solution.
C1V1 = C2V2
0.100 x C1 = 0.00500 x 500.0
V1 = (0.00500 x 500.0) ÷ 0.100
= 25.0 mL (3 sf)
(2 marks)
(ii) Name the two pieces of volumetric glassware used to prepare 500.0 mL of diluted
solution.
pipette
volumetric flask
(2 marks)
(iii) Identify the glassware that must be rinsed with distilled water only, and state the reason
why this glassware must not be rinsed with the original solution of KHP.
Analysis and
Evaluation
Analysis of
procedures is
accurate. The
reasoning behind
(c) In one investigation the KHP solution was delivered from a burette. The table below shows various
procedures (e.g.
the results of several titrations:
use of volumetric
glassware) is
trial
titre (mL)
explained
1
19.75
logically.
Volumetric flask. If it were rinsed with the KHP then there would be some
extra moles of KHP present in the diluted solution.
(2 marks)
2
3
4
5
18.45
18.50
18.50
18.45
(i) Explain one advantage of repeating the titration several times.
It would identify the presence of any systematic errors and so give some
information on how close the result is to the true value. (2 marks)
(ii) Determine the average titre, showing your reasoning.
Analysis and
Evaluation
Purpose of
repetition not
understood.
Data evaluated
and used
correctly.
Ignore 19.75 mL as the other four titre value are all concordant but the first one
is very different.
average titre = ¼ (18.45 + 18.50 + 18.50 + 18.45)
= 18.475
= 18.48 mL (4 sf)
(3 marks)
(iii) State the effect (increase/decrease/ stays the same) on the average titre of rinsing the
burette with distilled water immediately prior to the titration.
Increase.
(1 mark)
TOTAL: 15 marks
Question 3
Page 3 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
In order to test for the presence of Pb in a soil, samples of the soil were treated with HNO 3 and the
solutions analysed using AAS.
A calibration graph was obtained by determining the absorbance of standard solutions of Pb in
HNO3.
(a) The solutions used in the calibration were all prepared by dilution of a standard solution.
Calculate the concentration, in ppm, of the solution formed when 0.0050 g Pb is dissolved in
50.0 mL of HNO3.
ppm = mg/L
= (0.0050 x 103 mg) ÷ (50.0 x 10-3 L)
= 100 ppm (3 sf)
(2 marks)
(b) (i) The results of the calibration are shown in the table below:
Concentration of Pb (ppm)
0
2
5
8
Absorbance
0
0.15
0.37
0.60
On the grid below draw a graph to display these results:
Investigation
Graph is
constructed with
correctly
oriented and
labelled axes,
and appropriate
scale and line of
best fit.
absorbance
0.6
0.4
sample
0.2
0
4
8
concentration of Pb (ppm)
(ii)
(4 marks)
In one analysis the test sample gave an absorbance reading of 0.30. Use the calibration
graph above to determine the concentration, in ppm, of lead in the sample tested.
4 ppm
(1 mark)
(iii) In another analysis, the concentration of lead in the sample solution was found to be
6.6 ppm.
(1) State this concentration as ppb.
6600 ppm
(1 mark)
(2) The solution was prepared by treating 2.50 g of the soil with HNO 3 and making the
solution up to 250.0 mL.
Page 4 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
(A) Determine the mass of lead in the 250.0 mL of solution.
In the test solution: Concn (Pb) = 6.6 mg L1 in 250 mL
Thus, mass Pb in 250 mL solution = 6.6 x 0.25
= 1.65 mg
(2 marks)
(B) Hence determine the concentration, in ppm, of lead in the soil.
mass Pb in the 2.5 g soil sample = mass Pb in 250 mL solution
= 1.65 mg
Thus, concn Pb in soil = mgkg
= 1.65/2.5 x 103
= 660 ppm (3 sf)
(3 marks)
(b) The soil samples also contained other toxic metals such as Cd.
State and explain the effect, if any, of these other metals on the AAS analysis for Pb.
The other metals will have no effect on the analysis.
A lead lamp is used which emits a unique set of wavelengths that can only be
absorbed by lead. The other metals absorb a different set of wavelengths.
(3 marks)
TOTAL: 16 marks
Page 5 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
Question 4
(a) The diagram below represents a simple high performance liquid chromatograph (HPLC).
sample injected into
the mobile phase
mobile phase
under pressure
column containing a
polar stationary phase
computer
display
detector
waste
An analysis was carried out of a tablet that can be taken to reduce the symptoms of the common
cold. A solution was prepared of the tablet in a solvent and a sample of the solution was injected
into the column. The chromatogram below was obtained:
X
0
2
4
6
8
10
12
retention time (min)
14
15
(i) State the meaning of the term retention time.
The time taken for a substance to travel through the stationary phase.
(1 mark)
(ii) (1) On the chromatogram above, label with an X the peak that corresponds to the least
polar component of the tablet.
(1 mark)
(2) Explain your answer.
Page 6 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
The substance marked X had the shortest retention time i.e. it took the shortest
time to travel through the stationary phase. This means it travelled fastest and so
was the least strongly attracted to the polar stationary phase and so has to be the
least polar.
Analysis and
(iii) One of the peaks on the chromatogram is due to the solvent.
Suggest a procedure to determine which of the peaks is due to the solvent.
(3 marks)
Evaluation
Experimental data
from HPLC is
analysed perceptively
and logically and
connected with
concept to formulate
appropriate
conclusion.
Using the same stationary and mobile phases repeat the experiment but only run
the solvent through the column. Then compare the retention times to identify
which peak is the solvent as they will be the same.
Application
(3
marks)
Wellreasoned
solution to
problem.
Page 7 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
(b) Thin layer chromatography (TLC) can be used to separate and identify amino acids present in
a protein. An example of a chromatogram is shown below, together with a table of R f values
for some amino acid standards run with the stationary and mobile phases used in the
experiment.
solvent front
-6
-4
Y
-
amino acid
alanine
glycine
histidine
leucine
Rf
0.375
0.275
0.125
0.563
amino acid
lysine
serine
threonine
valine
Rf
0.163
0.213
0.300
0.500
-2
origin
-0
(i) Suggest why, although there were actually six amino acids present in the sample, there
are only five dots on the chromatogram.
Two of the amino acids most likely had very similar retention times. Therefore
they ended up so close to each other that it looks like one dot. One of the
dots is much bigger than
the others so this is a likely one.
Analysis and
Evaluation
(2Experimental
marks)
(ii) Identify, with reasoning, the amino acid most likely to be Y.
Y will be the acid with the same Rf value.
Rf = distance travelled by Y/ distance travelled by the solvent
data from TLC is
analysed
perceptively and
logically and
connected with
concept to
formulate
appropriate
conclusions.
= (3.5-1.0)/(6.0-1.0)
= 2.5/ 5.0
= 0.50
Y is most likely to be valine
(3 marks)
TOTAL: 13 marks
General Comments
Application
Apart from one instance of incorrect use of significant figures, appropriate terminology and conventions, and correct formulae
and equations have been used throughout the task.
Knowledge and understanding
Mostly correct and well-reasoned responses demonstrate a deep understanding of the range of concepts covered in this task.
Page 8 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
A
Investigation
Analysis and Evaluation
Application
Knowledge and
Understanding
Designs logical, coherent, and
detailed chemistry investigations.
Critically and systematically
analyses data and their
connections with concepts, to
formulate logical and perceptive
conclusions and make relevant
predictions.
Applies chemistry concepts and
evidence from investigations to
suggest solutions to complex
problems in new and familiar
contexts.
Consistently demonstrates a deep
and broad knowledge and
understanding of a range of chemistry
concepts.
Critically and logically selects
and consistently and
appropriately acknowledges
information about chemistry and
issues in chemistry from a range
of sources.
Manipulates apparatus and
technological tools carefully and
highly effectively to implement
well-organised safe and ethical
investigation procedures.
Critically and logically evaluates
procedures and suggests a
range of appropriate
improvements.
Uses appropriate chemical
terms, conventions, formulae,
and equations highly effectively.
Uses knowledge of chemistry
perceptively and logically to
understand and explain social or
environmental issues.
Demonstrates initiative in
applying constructive and
focused individual and
collaborative work skills.
Uses a variety of formats to
communicate knowledge and
understanding of chemistry coherently
and highly effectively.
Applies chemistry concepts and
evidence from investigations to
suggest solutions to problems in
new and familiar contexts.
Demonstrates some depth and
breadth of knowledge and
understanding of a range of chemistry
concepts.
Uses appropriate chemical
terms, conventions, formulae,
and equations effectively.
Uses knowledge of chemistry logically
to understand and explain social or
environmental issues.
Applies mostly constructive and
focused individual and
collaborative work skills.
Uses a variety of formats to
communicate knowledge and
understanding of chemistry coherently
and effectively.
Applies chemistry concepts and
evidence from investigations to
suggest some solutions to basic
problems in new or familiar
contexts.
Demonstrates knowledge and
understanding of a general range of
chemistry concepts.
Obtains, records, and displays
findings of investigations using
appropriate conventions and
formats accurately and highly
effectively.
B
Designs well-considered and
clear chemistry investigations.
Logically selects and
appropriately acknowledges
information about chemistry and
issues in chemistry from different
sources.
Manipulates apparatus and
technological tools carefully and
mostly effectively to implement
organised safe and ethical
investigation procedures.
Clearly and logically analyses
data and their connections with
concepts, to formulate consistent
conclusions and make mostly
relevant predictions.
Logically evaluates procedures
and suggests some appropriate
improvements.
Obtains, records, and displays
findings of investigations using
appropriate conventions and
formats mostly accurately and
effectively.
C
Designs considered and
generally clear chemistry
investigations.
Selects with some focus, and
mostly appropriately
acknowledges, information about
chemistry and issues in
chemistry from different sources.
Manipulates apparatus and
technological tools generally
carefully and effectively to
implement safe and ethical
investigation procedures.
Analyses data and their
connections with concepts, to
formulate generally appropriate
conclusions and make simple
predictions, with some
relevance.
Evaluates some procedures in
chemistry and suggests some
improvements that are generally
appropriate.
Uses generally appropriate
chemical terms, conventions,
formulae, and equations with
some general effectiveness.
Applies generally constructive
individual and collaborative work
skills.
Uses knowledge of chemistry with
some logic to understand and explain
one or more social or environmental
issues.
Uses different formats to
communicate knowledge and
understanding of chemistry with some
general effectiveness.
Obtains, records, and displays
findings of investigations using
generally appropriate
conventions and formats with
some errors but generally
accurately and effectively.
Page 9 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013
D
Investigation
Analysis and Evaluation
Application
Knowledge and
Understanding
Prepares the outline of one or
more chemistry investigations.
Describes basic connections
between some data and
concepts, and attempts to
formulate a conclusion and make
a simple prediction that may be
relevant.
Applies some evidence to
describe some basic problems
and identify one or more simple
solutions, in familiar contexts.
Demonstrates some basic knowledge
and partial understanding of
chemistry concepts.
Selects and may partly
acknowledge one or more
sources of information about
chemistry or an issue in
chemistry.
Uses apparatus and
technological tools with
inconsistent care and
effectiveness and attempts to
implement safe and ethical
investigation procedures.
For some procedures, identifies
improvements that may be
made.
Attempts to use some chemical
terms, conventions, formulae,
and equations that may be
appropriate.
Attempts individual work
inconsistently, and contributes
superficially to aspects of
collaborative work.
Identifies and explains some
chemistry information that is relevant
to one or more social or
environmental issues.
Communicates basic information to
others using one or more formats.
Obtains, records, and displays
findings of investigations using
conventions and formats
inconsistently, with occasional
accuracy and effectiveness.
E
Identifies a simple procedure for
a chemistry investigation.
Identifies a source of information
about chemistry or an issue in
chemistry.
Attempts to use apparatus and
technological tools with limited
effectiveness or attention to safe
or ethical investigation
procedures.
Attempts to connect data with
concepts, formulate a
conclusion, and make a
prediction.
Acknowledges the need for
improvements in one or more
procedures.
Identifies a basic problem and
attempts to identify a solution in
a familiar context.
Demonstrates some limited
recognition and awareness of
chemistry concepts.
Identifies some chemical terms
or formulae.
Shows an emerging understanding
that some chemistry information is
relevant to social or environmental
issues.
Shows emerging skills in
individual and collaborative work.
Attempts to communicate information
about chemistry.
Attempts to record and display
some descriptive information
about an investigation, with
limited accuracy or
effectiveness.
Page 10 of 10
Stage 2 Chemistry annotated response
Ref: A107228 (revised January 2013)
© SACE Board of South Australia 2013