Download Student 2 response

Stage 2 Chemistry
Assessment Type 2: Skills and Applications Tasks
Analytical Techniques Test
Student 2 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 rinse with water to remove any unwanted substances from the
burette.
Application
Step 2: Then rinse with the NaOH.
(b) Calculate the number of moles of HCl initially added to the crushed eggshells.
Solves simple
quantitative
problems in a
standard
volumetric
analysis context.
n = C.V
= 0.300 x 0.025
= 0.0075 mol
(2 marks)
(c) Calculate the number of moles of NaOH required to neutralise the HCl in Step 2.
n = 0.200 x 0.0182
= 0.00364 mol
(2 marks)
(d) Hence calculate the number of moles of HCl that remained unreacted at the end of Step 1.
0.00364 mol
(1 mark)
(e) Hence calculate the number of moles of HCl that reacted with the eggshells.
0.0075  0.00364 = 0.00386
(1 mark)
1 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (revised January 2013)
© SACE Board of South Australia 2013
(f) Calculate the percentage, by mass, of calcium carbonate in the eggshells.
(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
(ii) Explain the effect that this contamination would have had on the titre value.
The titre would have been more basic because KOH is basic
Application
Identifies a
familiar source
of(1
systematic
mark)
error but did not
solve a
subsequent
problem in an
unfamiliar
(2 marks)
context.
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 = CV
= 0.100 x .050 = 0.005
m = n/M
= 0.005/ 165.12
= 3.028 gms
(3 marks)
2 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (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.
n = C.V
(2 marks)
(ii) Name the two pieces of volumetric glassware used to prepare 500.0 mL of diluted
solution.
volumetric flask and a teat pipette.
(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.
Volumetric flask. This is because it would affect the concentration
and so it would affect the results.
(2 marks)
(c) In one investigation the KHP solution was delivered from a burette.
The table below shows the results of several titrations:
trial
1
2
3
4
5
Analysis and Evaluation
Limited analysis of
procedures.
titre (mL)
19.75
18.45
18.50
18.50
18.45
(i) Explain one advantage of repeating the titration several times.
It would reduce the random errors so that the results are more precise.
(2 marks)
(ii) Determine the average titre, showing your reasoning.
average titre = ¼ (18.45 + 18.50 + 18.50 + 18.45)
= 18.475
(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.
Decrease.
(1 mark)
TOTAL: 15 marks
3 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (revised January 2013)
© SACE Board of South Australia 2013
Question 3
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 0.0050g Pb is dissolved in
50.0 mL of HNO3.
n = 0.0050/207.2
(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
Scale is
appropriate,
points are
plotted correctly
and an
appropriate line
of best fit is
drawn. Axes
oriented and
labelled
incorrectly, and
units are
omitted.
8
concentration
6
4
2
0.15
0.3
0.45
0
Absorbance
(ii)
0.6
(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.
6.6 x 10-3 ppb
4 of 10
(1 mark)
Stage 2 Chemistry annotated response
Ref: A121058, (revised January 2013)
© SACE Board of South Australia 2013
(2) The solution was prepared by treating 2.50 g of the soil with HNO3 and making the
solution up to 250.0 mL.
(A) Determine the mass of lead in the 250.0 mL of solution.
mass of Pb =
(2 marks)
(B) Hence determine the concentration, in ppm, of lead in the soil.
(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.
Pb has a different wavelength than Cd So the Cd won’t affect the
analysis as Cd has a different wavelength.
(3 marks)
TOTAL: 16 marks
5 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (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 the substance to go through the column.
(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)
6 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (revised January 2013)
© SACE Board of South Australia 2013
(2) Explain your answer.
It has travelled the longest distance and so is the weakest absorbed and so is
the least polar.
(3 marks)
(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.
Application
Inadequate
description of
procedure.
Put the solvent through and it’s the biggest peak.
(3 marks)
7 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (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.
As two of the acids might have the same Rf values.
-
(2 marks)
(ii) Identify, with reasoning, the amino acid most likely to be Y.
Rf =
3.5/ 6
= 0.58
Y is most likely to be leucine
Analysis and
Evaluation
Correct formula
but incorrect
values used.
Appropriate
conclusion
made.
(3 marks)
TOTAL: 13 marks
General Comments
Application
Generally uses appropriate terminology. Some units have been omitted and incorrect symbols for units have been used.
Simple formulae have generally been used correctly.
Knowledge and understanding
Responses demonstrate a general knowledge and understanding of Chemistry concepts.
8 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (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.
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.
Applies chemistry concepts
and evidence from
investigations to suggest
solutions to complex problems
in new and familiar contexts.
Uses appropriate chemical
terms, conventions, formulae,
and equations highly
effectively.
Demonstrates initiative in
applying constructive and
focused individual and
collaborative work skills.
Uses knowledge of chemistry
perceptively and logically to
understand and explain social or
environmental issues.
Uses a variety of formats to
communicate knowledge and
understanding of chemistry
coherently and highly effectively.
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.
Applies chemistry concepts
and evidence from
investigations to suggest
solutions to problems in new
and familiar contexts.
Uses appropriate chemical
terms, conventions, formulae,
and equations effectively.
Demonstrates some depth and
breadth of knowledge and
understanding of a range of
chemistry concepts.
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 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.
9 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (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.
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.
Applies some evidence to
describe some basic problems
and identify one or more
simple solutions, in familiar
contexts.
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.
10 of 10
Stage 2 Chemistry annotated response
Ref: A121058, (revised January 2013)
© SACE Board of South Australia 2013