Question

EXPERIMENT 8 Spectrophotometric Study of an Equilibrium Reaction Since known amounts of iron(III) and thiocyanate will be mixed, and the concentration of the iron-thiocyanate complex determined spectrophotometrically, it will be possible to cal- culate the equilibrium constant for each of the three possible equilibrium reactions above. The reaction which gives a constant value of K, for all the solutions prepared will be the correct one under the experimental conditions. AGRERERERLER . 1 Procedure 1. Clean and dry five 125 x 15 mm (Large) test tubes. Label them 1-5. 2. Using the following table as a guide, dispense the correct amount of each solution from the burets in the hood into the appropriate test tube. Be as accurate with your volumes as possible, as deviations will affect your results. TEST TUBE VOL 0.00200 M IRONH) VOL 0.00200 M KSCN VOL 0.5 M HNO, NITRATE 5.00 mL 2.00 ml 3.00 mL 2 5.00 mL 3.00 mL 2.00 mL 3 5.00 ml 15ml 3.50 mL 1.50 ml 5.00 ml 4.00 mL 1.00 mL 5 5.00 ml 4.50 mL 0.50 ml 3. Stir the first solution using a clean dry stirring rod. Rinse the rod, dry with Kimwipes. and stir the second solution. Stir all five solutions, making sure to rinse and dry the stir rod in between 4. Take your test tube rack, Kimwipes, a wash bottle, waste beaker, and your lab manual and pen to the spectrophotometer. The wavelength should be set to 460 nm, and the "blank" cell should be filled with 0.002 M KSCN. 6644 Figure 8-1. Line up the white etch mark with the line on the cell holder. 5. Following the instructions for the spectrophotometer, "zero" the instrument for 0% and 100% transmittance using the "blank" sample. When inserting the special cells (called cuvettes) into the sample holder, make sure the etching on the cell lines up with the mark on the holder. The cell should be filled only halfway. Be sure to wipe the outside with a kimwipe and to handle the cell on the top half only. 112-82

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Answer 1

1. In order to approach such questions, one must first understand what is a transmittance. When light passes through a medium, the ratio of the intensity of the transmitted light to the intensity of the incident light is referred to as the transmittance. And the negative logarithm of this quantity is called the absorbance.

Since we know that the transmittance from the sample is 59.3 percent, we can use the following formula to ascertain the absorbance:

Absorbance = -log (Transmittance)

Now here we just have to plug in the given value:

Absorbance = -log 59.3 100

Absorbance = 0.227

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