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Spectrophotometry Lab Questions

  1. In graph (a), as the concentration increases, the absorbance increases

  1. In graph (b), as the concentration increases, the percent transmittance decreases

  1. If someone were to give you a solution of CuSo4, and to tell you that it had an absorbance of 0.1(or a percent transmittance of 90%), could you calculate the concentration of the solution? If you could how would you do it? Yes you could calculate the concentration of the solution by using the formula A=ebc. It states that A is absorbance (2 – log%T), e is the molar absorbtivity with units of L mol-1 cm -1, b is the path length of the sample – that is, the path length of the cuvette in which the sample is contained (cm), and c is the concentration of the compound in solution, expressed in mol L-1

As a result, you could calculate the concentration using the formula c=A/eb.

Would it be easier to use the percent transmittance reading or the absorbance in doing this calculation? Why? The linear relationship between concentration and absorbance is both simple and straightforward, which is why it is easier to use the absorbance reading in doing this calculation.

  1. What wavelength did you set the spectrophotometer to?  Why?  What do you think would happen if another wavelength were to be used? In the second activity of this lab, the wavelength was set to 640nm at the point of maximum absorption that was established in the first activity. This is because in order to test the concentrations of copper sulfate, the spectrophotometer must be set so there is a minimal amount of transmitted light. Materials absorb only some of the wavelengths of ordinary light but not others that give substances color. 640nm is the best absorbance wavelength of color for copper sulfate. If another wavelength were to be used, we believe the observations would be very inaccurate because the wavelength used would not be suitable based on absorbance.


  1. In this experiment, you have demonstrated that the spectrophotometer can be utilized to measure the absorbance of a solution and that the value obtained can be used to find the concentration of the solution.  Suggest a practical application for this technique. This technique can be used to analyze waste water, foods, and drugs.  Also, it can be used to measure the quality of colored products like fabrics, paints and plastics.


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