Beer's Law

Topics: Concentration, Light, Wavelength Pages: 12 (1872 words) Published: May 27, 2014
Beer’s Law Lab Report
The Beer’s law lab was conducted to determine the optimal wavelength of Co(NO3)2·6H2O with the use of spectrometry. The results determined that the optimal wavelength to study the absorbance of this salt was 500nm. It also demonstrated how transmittance of light and absorbance of light are inversely proportional because absorbance is calculated by multiplying transmittance by a negative log. Introduction:

When one is studying chemicals, there are many important factors of significance. The color of a chemical is a useful tool in its study. The light one sees produced by a chemical is the result of both reflection and absorbance of wavelengths. The wavelengths that are absorbed by a chemical are not visualized. The wavelengths that are reflected back are the colors that one sees. When chemicals are diluted in water, their colors also become diluted. As the chemical is diluted, the molecules spread apart. The more dilute the solution, the further apart the molecules. As the molecules spread, the color that is reflected becomes less intense because some of the wavelengths are able to pass through the solution without encountering any of the solute. The more wavelengths that are able to pass through a solution without encountering any of the solute, the greater the transmittance. The transmittance can be mathematically calculated by dividing the amount of light that exited the solution (IT) by the amount of original intensity (IO). That value is then multiplied by 100 to give the percent transmittance (%T)

Beer’s Law is used to relate and compares the amount of light that has passed through something to the substances it has passed through. The Law is represented by A=abc. “A” is the absorbance of a solution. The “a” represents the absorption constant of the solution being tested. The “b” represents the thickness of the solution in centimeters, and “c” represents the solution’s molarity or concentration. The “A” can be calculated by using the negative log of the transmittance (T).
The lab experiment conducted used the salt Co(NO3)2·6H2O. The Co(NO3)2·6H2O was diluted in distilled water to four different molarities. The most concentrated solution was used to determine the optimal wavelength to study the salt by measuring the transmittance of the Co(NO3)2·6H2O with twenty different wavelengths of light. Once the optimal wavelength was concluded, the transmittance of the less concentrated Co(NO3)2·6H2O solutions was also measured. The measurements of the less concentrated solutions was to determine the absorbance constant, “a”. Finally, the transmittance of an unknown concentration of Co(NO3)2·6H2O solution was measured and molarity was determined based on the absorbance constant determined earlier in the experiment.

A test tube was prepared with 0.1 M solution of Co(NO3)2·6H2O in 10mL of distilled water. Half of the .1M solution, 5mL, was drawn up into a pipette and put into another test tube with 5mL of deionized water to make a 0.05 M solution. Half of the 0.05 M solution, 5mL was drawn into a pipette and put into a test tube with 5mL of deionized water to make 0.025 M solution. Half of the 0.025 M solution, 5mL, was drawn into a pipette and put into a test tube with 5mL of deionized water to make 0.0125 M solution. A test tube of 10mL of deionized water was also prepared. The bubbles on all test tubes were removed by tapping on the outside of the test tube. The outside of the tubes were dried off and any fingerprints were removed with paper towels and placed into a test tube rack.

An absorbance spectrometer was zeroed by measuring the transmittance at 400nm with no test tubes in the spectrometer. The spectrometer was then calibrated to 100 percent transmittance with the test tube of deionized water. The deionized water was removed from the spectrometer...
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