Standard Solutions of Copper.  Each pair of students makes up one standard CuSO₄ solution.
Concentrations should be 0.13M, 0.11M, 0.090M, and 0.070M (0.050M, 0.15M are optional) as assigned by the instructor. Use 50 ml volumetric flasks and solid CuSO₄ × 5 H₂O. Weigh the copper sulfate on the analytical balances and record all digits.  Take care to transfer all the copper salt into the volumetric flask.  Add about 10 mls of DI water and then 20 drops of 6 M sulfuric acid to get the salt into solution.  Then dilute to the mark with DI water.  Dilute exactly to the mark; the bottom of the menicus must touch the reference line.  Start over (cheerfully) if you have any doubt that you made up the solution correctly.  Follow the same procedure for the unknown copper salt. All groups must work together to make one solution from the unknown copper salt using (of course) a carefully weighed amount of the salt.  The instructor will provide copper unknown solutions A and B. 

Using the Spec 20.  Each pair should have a digital Spec 20. Examine the colors of light produced by the Spec 20 at 700 nm, 650 nm, 600 nm, 550 nm, 500 nm, 450 nm, and 400 nm. This can be done by placing a strip of white paper in a test tube and inserting the tube into the Spec 20. The tube opens the shutter so that the light will hit the paper. The light is dim, so the room lights should be out while the you look into the Spec 20.

Adjusting the Spec 20. Use the left knob to make the Spec 20 read 0% T with no tube present (shutter closed, no light hitting the detector). Use the right knob to adjust for 100% T with a water blank (shutter open, maximum light hitting the detector).

Finding the best wavelength.  Using solution A, measure the absorbance at different wavelengths. Every time the wavelength is changed, the Spec 20 must be readjusted for 0% T and 100% T. Each pair writes their results on the board. The class then picks the best wavelength for analysis. What color is this wavelength? Why?  
Alternatively (and faster!)  Use an Ocean Optics spectrometer to determine the best wavelength for analysis.

Measurements.  Standard solutions are shared among the pairs; each pair makes a calibration curve using data from just one Spec 20 (don't mix data from different spectrometers!).  Each group records data for unknowns A and B, and the unknown copper salt solution.  You should record about four or five absorbances for each solution to assess the reproducibility of the measurement.  As you work with a sample, switch back and forth between the copper sample and the water blank, making sure that the water blank always reads 100% T.  (Adjust the right knob until it does.) 
Write your best absorbance for each known solution on the board for all to see!  Do the same for solutions A and B.

Analysis.  Put all measurements for knowns into a Excel spreadsheet with concentrations in one column and absorbances in another.  Do not average the absorbances for a single known solution; instead, put all the measurements into the graph.  Graph the data.   Try fits with and without a y-intercept. Be sure to plot the fit line through the data points. 
Compare the fits that you get in Excel using Excel's LINEST function, its REGRESSION function in the Data Analysis Toolpak, and the Trend Line feature available for charts.  All these methods should give the same equation for a line, but some methods are more helpful because the results can be used elsewhere in the spreadsheet.  The use of Trend Line alone is strongly discouraged!  In other words, don't stop fitting simply because you have a trend line on your graph.
In the Excel spreadsheet determine the concentrations of unknown solutions A and B; use equation 4-15 in Harris’s textbook to estimate the error in the concentration of the unknowns.
In the spreadsheet perform the calculations necessary to find the % Cu in the unknown copper salt, and of course estimate the error or uncertainty in the % Cu. Then each pair should try to identify the unknown copper salt using their value of the % Cu.  The Merck Index lists many copper salts and gives the % Cu of the salt, so the Merk Index is especially helpful as you explore possible identities for the unknown.  The error in the % Cu is important because it will help you decide what salts are possible and which can be ruled out by your experiment.