Full Lab Manual
Introduction & Goals
Chemistry & Background
In Your Write-up
This week you will analyze the compound that you synthesized last week. You will first determine the weight percent of cobalt in your sample by colorimetry. Secondly, you will determine the weight percent of water by observing the change in sample mass when the water is removed by heat.
To Learn or Review
Transition Metal Complexes
Oxtoby, pp. 345-347
Oxtoby, pp. 328-329
Colors of Complex Ions
Zumdahl, pp. 940-941
In this week's experiment you will use Beer's Law to analyze your cobalt complex, in an experiment similar to the Koolaid experiment of Week 4. Beer's Law should now be clearly understood, but we will repeat the basics here. Recall that the absorbance of light by a sample depends on the concentration of absorbers, the path length of the light through the sample, and the extinction coeffiecient:
Recall that the extinction coefficient, , is a proportionality constant between absorbance and concentration, for a given pathlength. A high value means that a substance absorbs light strongly at the particular wavelength. A plot of (or A with L and c specified) versus wavelength
() is known as the absorption spectrum of a substance. This spectrum shows which wavelengths of light are absorbed and which are not.
Figure 1 contains an experimentally measured visible absorption spectrum of aquapentaamminecobalt(III). It was measured on a recording spectrophotometer which continuously varies the wavelength of the incident light beam and records the absorbance as a function of wavelength in a computer file. The measured absorbance at any given wavelength is automatically corrected for the absorbance of the blank. From the data in Figure 1, the extinction coefficient at any desired wavelength can be calculated.
Visible absorption spectrum of [Co(NH3)5 H2O]3+
Sample concentration is 6.6 x 10-3 M and path length is 1.0 cm.
The blank is water.
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