Experiment you will be given a series of solutions

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Unformatted text preview: ght absorbed by the solution. The more light that is absorbed, the darker the solution is, and absorptivity is a measure of absorption. Absorption should not be confused with transmittance, which is the amount of light that gets through the solution (denoted by the symbol I for intensity), and is always measured relative to the amount of light entering the solution (denoted by the symbol I0 for the initial intensity of light entering the solution). Thus, we have transmittance, T= I I0 or, since transmittance is more commonly written as a percentage, we have percent transmittance, I %T = *100 I 0 Absorption can be shown to be related to transmittance logarithmically: A = log (T ) = log (%T ) − 2 We find that the relationship between absorptivity and concentration is linear. Beer’s law is written as Dakota State University page 205 of 232 Experiment 20: Beer’s Law General Chemistry I and II Lab Manual A = εlc where A is absorptivity, ε is the absorptivity coefficient which is a constant for any given chemical (it’s why one chemical is darker at the same concentration as another), l is path length of light through the solution (which we work very hard to keep constant during these studies so ε l=constant), and c is the concentration of the chemical. Because concentration is linear with absorptivity, and absorptivity is logarithmic with respect to percent transmittance, chemists, who are just as lazy a bunch as anybody else, much prefer using absorption for spectroscopic measurements so we don’t have to worry about working with logarithms in our calculations, despite the fact that it might be a little bit more intuitively simple to think of amount of light getting through the solution rather than the amount of light absorbed by the solution. Experiment: You will be given a series of solutions with known concentrations (at the time of the writing of this experiment, the exact substance we will use and the concentrations have not yet been determined), along with at least one solution with an unknown concentration. Because this is the first time you will have used the Pasco system, the instructions to follow are relatively complete (although you should also take this opportunity to explore the software); however, keep in mind that in future experiments, considerably less detail will be given. Be sure to learn the system as you use it. Starting and initiating the Pasco data collection system Starting the Pasco system: Obtain a colorimeter, and plug the jack into the “Analog channel A” plug. Turn on the Pasco “black box”, and start the computer. When the computer has booted, start the science workshop software package, found in the science workshop folder. Once the software has started, you will see a picture of the Pasco “black box” on the screen. Click and drag the picture of the analog jack on the screen to the picture of the “Analog channel A” plug; you will see a box highlighting the plug when you are on it. Release the mous...
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This note was uploaded on 09/18/2012 for the course CHEMISTRY 1010 taught by Professor Kumar during the Fall '11 term at WPI.

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