Monterey Peninsula College
CHEM 1B
Dr. R. Rinehart

Spectrophotometric Determination of an
Equilibrium (Formation) Constant

Instructions for using the workbook sent to you:

Helpful hints: "Save" file to C:\ and "Save" frequently while working. When you're done,
"Save As" to A:\ (your floppy). This should prevent the freezeup that some of you have previously experienced when saving everything directly to A:\

1) Print the instruction sheet (either at home or at school).
2) Copy and save this workbook to a 3.5" floppy and bring it to class tomorrow.
3) Look at the following worksheets in the order presented.

"wavelength" is where you will enter your % transmittance data that you recorded in part A of the experiment.
    a) Modify the header to put your name and a descriptive title for the worksheet on it
    b) Enter your data [number only, no units!] in the second column.
    c) In the first open cell (C2) in the third column, enter an = followed by a formula which
        will convert the %T entry in B2 to T. Use fill-in to complete the column.
    d) In the first open cell (D2) in the fourth column, enter an = followed by a formula
        which will convert T to A [refer to the definitions of T and A given in class].
        Use fill-in to complete the column.
       e) Hit the chart button and select "insert chart on same page" -- you will make an
        XY-scatter plot of the data in columns A and B.
    f) Hit the chart button again and select "insert chart on same page" -- you will now
        make an XY-scatter plot of the data in columns A and D.
    g) Before printing this page, please format the chart area to remove the default gray
        background. You may want to use other options to improve its appearance.

 "Beer-Lambert" is where you will enter your % transmittance data that you recorded in part B of the experiment.
    a) modify the header to put your name and a descriptive title for the worksheet on it
    b) enter your part B %T data [number only, no units!] in the second column.
    c) in the third column, enter the corresponding [FeSCN²⁺] in each tube
    d) in the first open cell (D2) in the fourth column, enter an = followed by a formula
        which will convert the %T in cell B2 to Absorbance
        [refer to the definitions of T and A given in class].
    e) hit the chart button and select "insert chart on new page" -- you will make an
        XY-scatter plot of the data in columns C and D.
    f) add a linear trendline. Have the chart show the equation and the correlation
        coefficient, since we will need the slope of that line for further calculations.
    g) Before printing this page, please format the chart area to remove the default gray
        background. You may want to use other options to improve its appearance.

 "Part C (Keq)" is where you will enter your % transmittance data that you recorded in part C of the experiment.
    a) modify the header to put your name and a descriptive title for the worksheet on it
    b) enter your part C %T data [number only, no units!] in the second column.
    c) in the third column, enter the corresponding A by using a formula and fill-in.
    d) in the fourth column, enter the INITIAL thiocyanate concentration for each tube.
    e) in the fifth column, enter the INITIAL concentration of Fe³⁺ in each tube.
    f) in the first open cell (F3) in the sixth column, enter the EQUILIBRIUM concentration
        of FeSCN²⁺. You can have Excel calculate this for you by setting up a formula using
        the absorbance in cell B2 and the extinction coefficient you obtained in part B,
        then filling in the formula for the rest of the column.
    g) You can now complete columns G and H by setting up Excel to subtract the value in
        column F from the values in columns D and E, respectively. (You hopefully realize
        at this point that columns D through H are a modified ICE table.)
    h) Set up Column I to calculate K_eq from the data in columns F, G, and H. Set up cell
        I10 to calculate the average of the K_eq's obtained above.

4) Both the completed datasheet and the chart should be submitted with your full writeup for this experiment. Your "results" section should summarize the essential findings of parts A, B, and C of this experiment -- "see attached spreadsheets" alone will NOT suffice!
Due date: Monday 7/7/03.

5) See the sites listed at http://www.mpcfaculty.net/ron_rinehart/1B/spectrok.htm  [this page]
for additional useful information that may help you in writing the introduction and/or discussion sections of your lab report.
 

©  Ronald W. Rinehart, 2003