Lab 10
Directions for Lab 10 Calculations Worksheet:
Some of the major takeaways from Lab 10 are to be able to utilize UV-Vis absorbance and Beer’s Law in order to calculate the concentration of Lawsone isolated from Henna. Complete the following attached Lab 10 Calculations Worksheet. The questions have no spacing, but you can download the Word Version to make room if you wish to type on this document. Please show all work.
Lab 10 Calculations Worksheet:
Your goal for this experimental problem is to determine the concentration of an unknown copper (II) sulfate solution. If you were doing this lab hands-on, you would be using a spectrophotometer. In a spectrophotometer, the amount of light that is absorbed by the solution can be measured. The CuSO4 solution used in this experimental problem has a deep blue color. A higher concentration of the colored solution absorbs more light (and transmits less) than a solution of lower concentration.
Five copper sulfate solutions of a known concentration (standard solutions) were prepared. These solutions are transferred to a small, rectangular cuvette that is placed into the spectrophotometer. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution.
The direct relationship between absorbance and concentration for a solution is known as Beer’s law. The concentration of an unknown CuSO4 solution is then determined by measuring its absorbance with the spectrophotometer. The concentration of the unknown can be found using the equation of the line of the Beer’s law curve. Below is the data that will be used to answer the following questions.
Trial Concentration (mol/L) Absorbance λmax (nm) Path Length (cm)
1 0.020 0.147 620 nm 1.2 cm
2 0.040 0.378
3 0.060 0.599
4 0.080 0.788
5 0.10 1.0
Unknown ? 0.446
1. What is the relationship between absorbance and concentration? What is happening at the particle level when the absorbance is either increased or decreased?
2. Create a graph of concentration vs. absorbance. Make sure the line of best fit is on there, along with the R squared value.
3. Based on the graph from question 2, what is the concentration of the unknown CuSO4 solution? Show all work.
4. Now we will use this data to calculate molar absorptivity at 620 nm. There are two ways this can be done.
a. Method one. Use Beer-Lambert’s Law equation to calculate molar absorptivity.
Write Beer’s Law equation here: ______________
Show work here:
Trial Concentration (mol/L) Absorbance Molar absorptivity (L mol-1 cm-1)
1 0.020 0.147
2 0.040 0.378
3 0.060 0.599
4 0.080 0.788
5 0.10 1.0
Unknown 0.446
b. Method two. Calculate the molar absorptivity using the line of best fit.
Write Line of Best fit equation here: ______________
Show work here:
Trial Concentration (mol/L) Absorbance Molar absorptivity (L mol-1 cm-1)
1 0.020 0.147
2 0.040 0.378
3 0.060 0.599
4 0.080 0.788
5 0.10 1.0
Unknown 0.446
5. Which method of calculating the molar absorptivity is better? Why?
6. What can the molar absorptivity tell us? What is the relationship between molar absorptivity and absorbance? Why is this important?
7. How is molar absorptivity related to wavelength?
8. Now let’s talk about dilutions. What is the purpose of a serial dilution? Here is an easy one: a chemist has 0.38M stock solution of HCl. How much stock solution is required to make 500mL of 0.26M HCl? Show all work.
9. Now let’s apply it to the experimental lab problem. You have 30mL of a 0.1M CuSO4 solution. Prepare the following concentrations using serial dilution:
Concentration How much stock is needed? (put units!)
0.05mM
0.04mM
0.03mM
0.02mM
0.01mM
0.005mM
Show work here:
10. Write a general explanation of the theory behind this experimental problem and the sources of error. Then describe a possible practical application of the methods used in this experimental problem. You may want to research UV-VIS absorption spectroscopy to get a better idea of the wide range of uses of this important analytical technique.