MHS AP Chemistry
Optical Spectrum Lab

A great deal of information can be learned from observing the frequencies a compound absorbs.  These frequencies can be observed using a tool called a spectrometer.  Depending on the wavelengths observed, a chemist may use a UV Spectrometer, and IR Spectrometer, or a (visible) Spectrophotometer.  This last tool is the one most often used in high schools because it easy to operate, and thousands of dollars cheaper than the others.  MHS currently owns four of these.

Instructions for using these machines are printed right next to the controls, so they will not be repeated here.

Part 1
The objective for this lab is to measure the % transmission of light through a dilute solution of one of the compounds listed below.  Record a table of wavelength (every 10 nm from about 360 nm) and % transmission (0-100).  Produce a graph with l on the x axis, and %T on the y axis.  Be sure to indicate which compound you examined.

After your graph your data, use the table to indicate which regions on your graph correspond to each color of the rainbow.  You can indicate the boundaries with dotted lines, or shade them with color pencils, or surprise me - keep it readable but have fun.

Part 2
Fill a test tube with the original solution you used.  Observe the absorbance at one or two of the frequency peaks you determined in Part 1.

Make a dilution of 5 mL of your solution + 5 mL of distilled water.  Record the new concentration and the absorbance(s) at the same frequency(s) as above.

Now dilute THIS solution in a 5:5 ratio.  Repeat the absorbance reading as above.  Repeat this step three more times.

Make a data table with the following headings:  Concentration, Absorbance.  Graph this data (hint:  which is the dependent variable?).

Pass In
Each person will pass in two graphs with the information above, two data tables, and the usual title stuff.  We may write this lab up electronically; in which case you can pass it in as an Excel Workbook as described here.

Use 0.1 or 0.5 M solutions of any of the following:
Ni(NO3)2 Cu(NO3)2 Fe(NO3)3 CoCl3
CoCl2 CuCl2 CuSO4 any ideas?

Colors, Wavelengths, & Frequencies
wavelength (l)
frequency (n)
400 to 420
7.5 to 7.1
420 to 490
7.1 to 6.1
490 to 580
6.1 to 5.2
580 to 590
5.2 to 5.1
590 to 650
5.1 to 4.6
650 to 700
4.6 to 4.3

[Special Graphing Hints for this lab][General Graphing Instructions][MHS AP chem page]