Week 6: Coordination Chemistry 2 Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Goals 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 Coordination Compounds Oxtoby, pp. 328-329 Colors of Complex Ions Zumdahl, pp. 940-941 Introduction 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: A = c L where       A = absorbance c = concentration of absorbing species in mol L-1 L = path length in cm = extinction coefficient in L mol-1cm-1 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. Figure 1 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. Example. Calculate the extinction coefficient, , for (Co(NH3)5 H2O)3+ at the long wavelength absorption maximum, 493 nm. A493 complex = 0.32 (read from the graph) = c L Beer's Law is useful in the analysis of solutions containing colored compounds, like your cobalt complex. At a particular wavelength, Beer's Law can be used to relate absorbance and concentration. Since A = c L , a plot of absorbance vs. concentration, for fixed and L, will be a straight line with slope L and intercept zero, as you saw in Week 4. This week you will use the same type of analysis to determine the concentration of cobalt complex in a solution you prepare
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