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In Your Write-up
1. Using data from Figure 2, calculate the absorbance and extinction coefficient for [Co(SCN)4]2- at the long wavelength maximum (625 nm). Compare this to the extinction coefficient of [Co(NH3)5 (H2O)]3+ in the example given earlier. Why is your analysis performed on [Co(SCN)4]2- rather than on [Co(NH3)5 (H2O)]3+?
2. Suppose that the procedure for colorimetric determination of cobalt were to be applied to the solid salt CoCl2. What size sample should be weighed out to produce a final absorbance of 0.340? Assume = 1.80 x 103 M-1 cm-1 for [Co(SCN)4]2- and L = 1.17 cm.
3. Calculate the % Co by mass in a sample of pure [Co(NH3)5 (H2O)][NO3]3. If your sample contains impurities that do not contain cobalt, will the % Co you determine by colorimetric analysis be higher or lower than this theoretical value? Is the colorimetric analysis sensitive to ligand-substitutional impurities, which contain cobalt?
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