Question

1. Make a scatter plot (on the computer again) of absorbance versus concentration for the calibration samples. On the plot, add a linear trendline and display the equation on the chart. Take your time to label the axes. Attach the chart to the end of this experiment. 2. From your plot, what is the value of eb?

Answer 1

The equation governing absorbance experiments in the Lambert-Beer law, which states:

$A=\varepsilon\cdot b\cdot c$

Where A is the absorbance, epsilon is the molar absorptivity, b is the optical path length and c is the concentration of the absorbing analyte. This equation can be considered as a lineal relationship between A and c, with a proportionality constante given by εb. In other words: the relationship between absorbances and their corresponding concentrations should yield a straight line with a slope corresponding to the value of εb. To get that line, you should plot our absorption-concentration pairs and perform a linear fit: (please take into account that commas are decimal separators):

We now face a problem, since the given data makes no sense, since absorbance should increase with increasing concentration, and the exact opposite is happening here. In order to complete the excercise, I'll show you the graphs:

The data is:

1,2 Absorbance Conc (M) 0,0007 1,1 0,000543 1,5 0,000409 1,5 0,00027 1,99 0,000139

And the graph we get from this data, with the linear fit, is:

Abs y=-1399x + 2,0347 R2=0,7922 O 0,0001 0,0002 0,0005 0,0006 0,0007 0,0008 0,0003 0.0004 Conc (M)

Which is quite bad as a linear fit, since R² should have a value much closer to 1, but much more serious is the fact that the slope (-1399) is negative, which doesn't make any chemical sense, since nor epsilon or b can take negative values. Please review your results!