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# Spectrophotometry

## Beer's Law: The Effect of Concentration

What are the factors that affect the amount of light a sample absorbs?

In this exercise, the role of the analyte concentration is examined. The analyte is the substance that absorbs the light. (Literally, it is the substance that is being analyzed in the experiment.) The concentration is represented by the symbol c and is typically measured in mole L-1.

In the experiment presented below, you will perform a series of simulations in which all experimental parameters are the same except for the analyte concentration. Each simulation will be performed using a different concentration to isolate the effect of the concentration on the absorption of light.

1. How does the concentration affect the intensity of light reaching the detector and why is this behavior observed?
2. How does the transmittance vary with the concentration? Is this plot linear?
3. How does the absorbance vary with the concentration? Is this plot linear?

### Experiment

For each simulation:

1. Select a concentration.
2. For each concentration, perform the simulation to determine the intensity of light that reaches the detector.
3. Calculate the transmittance.
4. Calculate the absorbance.
5. For each simulation, plot the c,T and c,A points on the graphs.
6. After all the points are plotted, carefully examine the plots to see how T and A vary with c. Is either plot linear?

Notes:

• The intensity of light from the light source is 500.0 photons/sec. (Only 10% of the photons are actually shown on the screen.)
• The transmittance is a fraction and is often expressed as a percentage. In plotting the results, enter T as percent transmittance.
• If possible, run the simulation sufficiently long to detect at least 1000 photons.
 Concentration: 0.00100 mole L-1           0.00200 mole L-1 0.00300 mole L-1           0.00400 mole L-1 0.00500 mole L-1 Number of Photons Detected: Elapsed Time: sec Intensity of Detected Light: photons/sec LightSource Cell Detector Plot Point:       c = mole L-1     T = %     A =

Effect of Cell Path Length                     Beer's Law