Set the wavelength on the spectrophotometer to lmax (determined in part II) using the Wavelength Selector dial.

  1. Reset the transmittance and absorbance of the Spectrophotometer as done in Part II Step 1.
  2. Measure the absorbances of all 6 solutions (5 knowns and 1 unknown) at lmax .
  3. Plot your results of Absorbance vs. concentration. Include this graph in your Report Sheet.
  4. From this graph of Absorbance vs. concentration, figure out the concentration of your diluted unknown and enter it into the Report Sheet Table in Part III.
  5. From the concentration of the diluted unknown and how you made this dilute solution (step 4 of Part 1), figure out the concentration of the original sample and record this on your Report Sheet Table, Part III.

(Show work on your Report Sheet for the calculation of Solution C and the unknown solution)


Calculate the concentrations of  the 5 samples (A-E) using the dilution formula M1V1 =M2V2.

Sample calculation –  Concentration of Solution A

Mi x Vi = Mf x Vf

Mf   =

Mi  = 0.50 M

Vi = 2.0 mL

Vf = 20.0 mL

Mf =

Mf = 0.050 M

Sample calculation – Concentration of cobalt(II) in the original unknown sample

10.5 mL of  an unknown cobalt(II)  sample was diluted to  30.0 mL.  The concentration of the diluted unknown sample was determined spectrophotometrically as in this experiment and was found to be 6.5 x 10-5 M. What is the concentration of cobalt(II) in the original sample?

Using the dilution formula again:  Mi x Vi = Mf x Vf , you are looking for Mi

Mi =

Mi  = ?

Vi = 10.5 mL

Mf = 6.5 x 10-5 M

Vf = 30.0 mL

Mi =

Mi =  1.9 x 10-4 M


  1. Plot A vs. wavelength – turn in graph.
  2. Determine the wavelength that has the maxium absorbance.


  1. Plot concentration vs. A – turn in graph.
  2. Determine the concentration of the unknown sample from your graph.



Instrument should be warmed up for 15 minutes.

Wipe all cuvettes with Kim Wipes to remove liquid droplets, dust and fingerprints before inserting into the instrument.

  1. Set wavelength control to desired wavelength.
  2. With sample compartment empty and the mode set to TRANSMITTANCE, set the display to 0% by using the lower LEFT hand knob. Once this is set, do NOT touch this lower left hand knob again.
  3. Switch the mode to ABSORBANCE. Insert clean cuvette that is about half full of deionized water. Adjust the display to 0% by using the lower RIGHT hand knob.
  4. Insert the sample and read the absorbance.

Colorimetry Report Sheet


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