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[crammy90]

when we have graphed the results from the standards and wish to work out a conc beyond what we have graphed (i.e. HSC 2006 25 where we have to extrapolate), do we do a line of best fit and then base our answer on that? or do we just estimate how the curve is changing?

 

[Azreil]

From the marking notes:

Question 25
(a)
Most responses correctly plotted all the values given. Better responses recognised that a curve of best fit was appropriate.
(b)
Better responses indicated the greater validity of interpolated estimates as opposed to those extrapolated from the graph. The following response demonstrates this.
Sample 1 – 13.5 ppm Sample 2 – 27.5 ppm The estimate at Sample 1 is quite valid because it is obtained from the line of best fit that is obtained by measuring absorbance at concentrations of copper ranging from 0 to 25 ppm. The measured absorbance and concentration lies within this range. However, estimate at Sample 2 cannot be considered to be as valid because the absorbance and thus the concentration lies outside the measure range. It assumes that the line of best fit sustains even for concentrations higher than 25 ppm. Hence, it is not as valid. Measuring the absorbance at 30 and 35 ppm of copper concentration would make the results more valid.

 

[danz90]

Well, simply.. you cant ensure your data is valid unless you have quantitative evidence. Therefore, by extrapolating.. you are only extending the trend of the graph, which would be invalid, as there would be no quantitative evidence to support your conclusion based on that extrapolation.