Exemplars of Student Work

1. Level 2 90694 (3.1) - Carry out an extended practical investigation involving quantitative analysis (Version 2) Exemplars of Student Work

2. Exemplar 4: Not Achieved • A report on the investigation has been presented along with a log book 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: • initial ideas for the purpose and what would be required for the investigation • analytical procedures and lists of chemicals and equipment required • detail related to intended investigation • trials and associated notes including problems encountered • information on chemicals being used • continued trials and calculations • final procedure used, data from all titration carried out. • The cover page of the report has not been included since this did not contain any further information relevant to the standard. • The key issue preventing Achievement is that the calculation associated with the analysis involved is incorrect resulting in an incorrect amount of substance being determined. The errors involved here are significant. • In addition: • statement of purpose is marginal • there is no summary of collected and processed data as is required for Achievement. The summary includes only the collected data with respect to values of the independent variable. The processed data is the amount of substance determined • data collected is not quality data, as required for Merit, since the EDTA solution used has not been standardised • discussion does not meet the requirement for Merit. • Note that as there is no reason given as to why the calcium content in milk would change when a sample is heated, a teacher may suggest the student consider a different purpose.

3. Report • Page 1 • Purpose is not particularly clear as it does not state how ‘heat’ is being applied. This could be differing lengths of heating time of heating or exposure to different temperatures. Further on it is evident that the length of time of heating at boiling point is the independent variable. • The purpose should be a short, concise statement. • Background information is mostly not relevant. There does not appear to be an understanding of the role of the potassium hydroxide solution in the analysis. Most of the background information (continued on page 2) is related to the procedure rather than the purpose of the investigation. • Relevant background material should be related to the reason (purpose) for doing the investigation. • Page 2 • A method is started on this page but it is a continued discussion related to why potassium hydroxide is to be used. • Page 3 • Describing a method and a procedure is a little confusing as they are the same thing. The final procedure used needs to be clearly stated, ideally step-by-step. This will include preparation of the sample as well as the analytical technique. The lack of clarity would make it difficult for the investigation to be duplicated by another person. • Page 4 • The term ‘amount’ is incorrectly used and this should be ‘volume’. These results are the average titres for the different heating times. Titration data in the log book shows the full data and concordancy of titres gained, for each time, is appropriate. • Graph again shows incorrect use of ‘amount’ and refers to a procedure line and a trend line. A procedure line is not meaningful. Rather than saying there is no trend, the data is showing no change in average titre. • Page 5 • The calculations are incorrect. A direct titration between the EDTA and calcium ions has been carried out. The average titre of EDTA used has not been used to determine amount of calcium ions as 50 mL has been used in this first step. This has negated the 50 mL of milk used in the next step so the assumed calcium ion concentration is the same as the concentration of the EDTA solution. In determining mg/L, of calcium the molar mass of EDTA has been used. • These errors cannot be considered minor. • Validation results are given but there is nothing to indicate what these actually are. • Page 6 • The conclusion is incorrectly interpreted as no trend. The trend is that there is no change in amount of calcium. • Discussion does not suggest why the milk was frozen. • Page 7 • Aspects in the discussion related to change of temperature during the day and temperature of heating - having a range of 2 degrees Celsius could be considered minimal. • Water was added to the milk before it was heated and there is a statement in the discussion saying that dilution with milk should be avoided. There is no rationale for this statement. Some evaporation of water will occur when the sample is heated and this has not been identified. While this will not be an issue for a specific heating time (since aliquot was measured after heating) a longer heating time will result in greater amount of evaporation.