CHEMISTRY 220.2 REPORT TEMPLATE
rev’d 8/20/09
Analysis of a Copper Salt using Absorption Spectroscopy
Name Your name here
Lab Partners’ Names Your partners' names here
Date of Experiment
Date Report Submitted
Abstract
An Ocean Optics spectrometer was used to measure the absorbance of a series of copper sulfate
solutions so that a Beer’s Law calibration curve could be plotted. The instrument was then used to
analyze three unknowns: two copper sulfate solutions and a copper salt. Solution A had a copper
concentration of

and solution B had a copper concentration of
percentage of copper in the unknown salt was


. The
; our best guess is that the identity of the salt
is chemical name.
Introduction
Since aqueous copper solutions are blue, they absorb what color? light. The goal of the experiment
was to analyze several copper unknowns by measuring absorbance. According to Beer’s Law, absorbance
is directly proportional to concentration.
Insert the equation for Beer's Law from MS Equation Editor. where A is the absorbance, ε is the
molar absorptivity, b is the path length through the solution, and c is the concentration.
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To use Beer’s Law, one usually measures the absorbance of a series of solutions containing different
concentrations of the absorbing species and plots the absorbance vs. concentration to obtain a calibration
curve. Then the concentration of the unknown solution can be read from the calibration curve once its
absorbance is measured. If the calibration data are fitted to a straight line A = mc + b where m is the
slope and b is the y-intercept, then the unknown concentration can be computed using C = (A – b)/m.
There is always some error in the use of a calibration curve to determine the concentration of an
unknown. If the calibration curve is a straight line based on a least-squares fit, the error in the unknown
can be computed using
Insert the error equation for a calibration curve from Harris's textbook.
where the x’s are the concentrations and the y’s are the absorbances
When we use a calibration curve to analyze an unknown, this equation tells us how precisely we have
determined the concentration of the unknown. That allows us to figure out how precisely we have
determined other quantities about the unknown, such as the percent copper. Knowing the range of
reasonable copper percentages helps us determine the identity of the unknown.
Experimental Method
Describe briefly how the spectrometer is operated, and describe the procedure you used to find the
best wavelength for the measurements. What was the wavelength? Use the passive voice and the past
tense.
Describe how you made the copper standard solutions and shared them. Be sure that the same amount
of acid was used to prepare all knowns and unknowns!
Describe how you handled the unknowns. What are the colors of the unknown solutions--do they look
like the copper sulfate solutions? What is the color of the unknown salt?
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Raw Data and Graphs
Table 1. Title for table 1. You can use this table for both standards and unknowns.
Solution
Concentration (M)
Absorbance
Insert Graph of Absorbance vs Concentration from Excel. Caption below the figure.
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Discussion and Error Analysis
Answer the following questions in present tense using paragraph organization. Assess the quality of
your calibration curve--is it straight? are there any bad points? Using the calibration curve, which
unknown solution, A or B, had the greater error/uncertainty in its concentration? Why? Is there a way
that you could improve the determination of that unknown? What is the error in the percentage copper of
your unknown salt? Attach a copy of your spreadsheet to the back of the lab report showing the
calculations of the unknown concentrations and their errors.
Conclusions
Answer the following questions in the present tense using paragraph organization. Your claim
is your identity of the unknown copper salt; explain how you arrived at its identity. Refer to your
data and data analysis, and make your warrant clear. You can use the color of the salt to help
identify it. You might not be able to make a definite identification; there may be two or even three
possible copper salts that you cannot rule out. That is okay. Do the best you can given the limited
amount of data. Explain how the estimated error in the copper percentage influences the process of
identification.
Literature Cited
You will also need to cite the error equation for a calibration curve that you get from Harris's
textbook. Finally, you should cite a source that contains a list of copper salts such as the CRC Handbook
or The Merck Index. I recommend the Merck because it will have the percentage of copper already
calculated for you; however, its list of copper salts will not be as complete as the CRC Handbook.
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