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Merit exemplar (PPT, 13MB)

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Level 2 90694 (3.1) - Carry out an
extended practical investigation
involving quantitative analysis
(Version 2)
Exemplars of Student
Work
Exemplar 3: Merit
A report on the investigation has been presented along with a comprehensive log book (49
pages) which includes the work carried out over a period of time.
The log book has not been included with the exemplar material but included the following:
•list of possible investigations
•plan and time frame
•possible methods for determining amount of iron
•trials carried out to test for iron(III) in spinach leaves
•research on iron in foods and reasons for changing to amount of iron(II)
•method of analysis using permanganate trialled
•trials involving reduction of iron(III) to iron(II) and then determining amount of
iron(II) in spinach
•related calculations
•final purpose and procedure decided with further trials and related calculations
•making and standardising permanganate solution
•preparation of sample
•results and calculations for all boiling times
•repeat of procedure with new samples and new standard
•all calculations
•comments reflecting on the investigation.
The cover page of the report has not been included since while this contained the purpose of
the investigation, it did not have any further information relevant to the standard.
The key issues that limit the judgement to Merit are:
•the error in processing of data as the percentage calculations are not correct. This has
prevented a discussion of what was actually happening during the analysis that was
carried out.
•using the procedure given, as the amount of iron(II) is much higher than would be
likely, the discussion needs to question the validity of this procedure. Something else
must have been oxidised by the permanganate solution. As the research provided
indicates that this may be due to the presence of oxalates in the spinach, the procedure
used needs to have been considered and discussed further..
Report Page 1
Purpose stated. Background material described. This considers iron content in spinach and
earlier related research. Also, information related to absorption by the body. Description of the
method that will be used and chemistry related to this.
Page 2
The procedure used is outlined. Preparation of sample is clearly outlined although
measurement of water added is not carried out accurately. Procedure does not state
concentration of sulphuric acid used although this can be found in the log book.
Page 3
Detail of the preparation and standardising of permanganate solution. This does not have to be
in the report and could be referenced to the log book.
Page 4
Sample calculation using collected data. The calculation is correctly carried out to determine
amount, and mass, of iron(II) in the 25 mL sample analysed. The equation involved in the
analysis is shown and reacting mole ratio correctly used.
However, the percentage calculation is incorrect. 100 g of spinach was cooked in 500 mL of
water and a 25 mL sample of this water contained 58.7 mg. Therefore the 500 mL of solution
(made from 100 g of spinach) would have contained 1.17 g iron(II). This is the iron(II) lost
from the spinach rather than what is contained in the spinach. Therefore, mass lost is much
larger than would be expected as it exceeds the mass of iron(II) found in spinach, according to
the research data.
Page 5
Calculation to determine concentration of the standard permanganate solution. This could
remain in the log book and be referenced.
Page 6
Summary of collected and processed data has been provided. Titration data in log book
shows concordancy of titres used to determine average titre. While the trend is unchanged, the
mass of iron(II) only relates to the 25 mL sample and percentage calculation is incorrect.
Page 7
The conclusion could be based on the amount of iron(II) in the water analysed and this is
increased as the boiling time is increased. That does mean the % of iron(II) in the water has
increased but that is not what has been calculated.
The discussion considers things that may have altered accuracy of result and compares data
gained to background information. If the previous calculation had been carried out to correctly
determine mass of iron lost from the whole 100 g sample, this would have led to questions about
the procedure carried out.
Page 8
Evaluation of the procedure used shows a repeat of the process that was carried out. Data
relating to this is in the log book and shows a new sample of spinach being prepared and a new
permanganate solution being prepared and standardised. A similar trend is found with the data
collected.
Pages 9 пЂ­ 11
Discussion on the reliability and accuracy of data related to the procedure carried out and
discussion of why modifications were made to the original procedure.
The presence of oxalates in spinach has been discussed and it is noted that oxalic acid will be
oxidised by permanganate and that this may have altered the results. While this is the likely
explanation for the impossibly high amount of iron(II) assumed to be present, the incorrect
calculation has meant this has not been fully considered.
Discussion related to both iron(II) and iron(III) and trials carried out to determine whether there
might be iron(II) in the spinach.
Page 12
A bibliography has been included.
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