Lab Handout Page

This virtual experience has been designed to allow a student to perform this lab without having access to the actual solutions and a spectrophotometer.

Purpose:

To investigate the absorption of light utilizing a series of solutions of Cobalt (II) Chloride ( CoCl2).

Using a VIRTUAL SPECTROPHOTOMETER.  You will learn how to: 
 - determine a lamda () max  (the wavelength at which the solution has the highest absorbance), 
 - make a standard curve, and 
 - find the concentration of an unknown using Beer's Law.

Procedure:  

Step 1. Number each of 7 dry 18 x 150-mm test tubes from 1 to 7.

Step 2. Pipette to make the additions of 0.150 M CoCl2  and distilled water shown in the table below.

Table 1. Dilutions of the 0.150M Stock Solution of Cobalt Chloride
Test Tube No.
Volume of 0.150 MCoCl2  Solution
(mL)
Volume of Distilled Water added
(mL)
1 5.0 0.0
2 4.0 1.0
3 3.5 1.5
4 3.0 2.0
5 2.5 2.5
6 2.0 3.0
7 1.0 4.0

Step 3. Thoroughly mix the contents of each test tube.

Step 4. Measure the Absorption of aqueous CoCl2  by using the 0.150 M CoCl2  .  Record the absorbance at intervals specified between 400 and 600 nm.

Step 5. Graph your data, wavelength and the x-axis; absorbance on the y-axis. From these measurements, select the wavelength at which absorbance is largest. ( Lambda Maximum ) max.

Step 6. Measure the absorbance of the contents of each of the 7 test tubes at this wavelength. Record your results.

Step 7. Using "Graphical Analysis" - plot your concentration data concentration on the x-axis, absorbance data on the y-axis.

Step 8. Using regression and statistics, determine the slope of the best straight line that satisfies these points.

Step 9. Measure the absorbance of the unknown.  Using the graph, Interpolate the concentration of  CoCl2  that corresponds to this absorbance.

 

Results:

Make a chart like the following:

1. The absorption data for CoCl2

Table 2. Absorbance Values at Each Wavelength in Nanometers
(nm)                Abs (nm)                Abs (nm)                Abs (nm)                Abs
400 480 504 540
420 485 506 550
430 490 508 560
440 495 510 570
450 498 515 580
460 500 520 590
470 502 530 600

 

2. Calculate the concentration in moles/L of the 7 samples prepared in Step2.

 

3. Standard curve.

    Wavelength to be used for measurements ( max): ______________nm

Table3. Calculated Concentrations from Table 1 Dilutions and Corresponding Absorbance
Test Tube Number Concentration mole/L Absorbance
1    
2    
3    
4    
5    
6    
7    

 

4.     Absorbance value of the unknown ____________

        Concentration via interpolation ____________

        Concentration via Calculation of Beer's Law __________

                                    A = kc                     A = Absorbance
                                                                    k  = slope
                                                                    c = Concentration

5a. Calculations for the Slope of your graph.

 

 

 

 

 

 

 

 b. Final determination

        Volume of unknown (mL): ____________________

        Volume of H2O (mL): _____________________

        A = _________________

        c = _________________ M