1) A student prepared a single solution and measured the %T and calculated the a
ID: 1082477 • Letter: 1
Question
1) A student prepared a single solution and measured the %T and calculated the absorbance. Then, the student plotted the single point and, using (0,0) as a second point, drew a Beer's Law plot.What potential problem might occur in doing this? 2) A second student decided to use % Transmittance in order to prepare a calibration plot instead of Absorbance. The student plotted %T vs. Concentration on Semi-log graph paper. Semi-log paper is linear on the x-axis and logarithmic on the y-axis. Can this student do this successfully? Explain why or why not 3.) On the next page is the result of a Beers'Law plot obtained in the lab. As can be seen, the Y-intercept is below zero, when Beer's Law indicates that it should be 0 Upon investigation, it was determined that the student used a common test tube instead of a cuvette to calibrate the spectrophotometer. He then used a cuvette for measuring his samples.Explanation / Answer
Ans: Beer Law: When light is felled on a solution, some part of light will get absorb and remaining part is transferred. The absorption of light is depending upon the concentration of solute present in the medium, allowed to pass the remaining light.
The amount of light incident and amount of light pass through the sample are related by relation of transmittance,
%T = It/I0 × 100 where It = transmitted light and I0 is incident light.
Percentage transmittance of light is related to the absorbance of solution
A = —log(%T/100)
However, the absorbance of the solution depends upon the concentration (c) of the solution, the pathlength of cuvette across which light traveled (l) and the molar absorptivity ().
Hence from Beer's law, the absorbance of the solution related to these three parameters by the relation. A= × l × c
Hence from this relation, we two parameters remain the constant during the experiment. Hence A only depends upon the concentration (c). So, the solution which follows the Beer's law, must follow this relation and we should have a linear graph. That's why we need to calibrate the graph with different concentration to get a linear fit of the line.
Solution: If take some point on the graph the linearity might be loose which results in wrong detection of the concentration. The results might be inaccurate.
There are several precautions which we have to considered while following this experiment.
No solute-solute interactions
No significant change in the refractive index with concentration of solute
No apparent chemical deviations (Shift in chemical equilibria etc)
Instrumental limitations as concentration increases
It might be possible that, any factor is keep on affecting the absorbance while performing the experiment.
Hence, by considering the linearity of the graph, by considering the two points results in wrong calibration of A vs c graph. Hence, results obtained might be incorrect.
Example calculation of molar absorptivity or concentration of an unknown solution.
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