0330prelabanswersexpt4F2011-1

0330prelabanswersexpt4F2011-1 - Experiment 4 What is the...

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Experiment 4: What is the Equilibrium Constant for Cobalt (II) Complex in Alcohol Solvents at Various Temperatures? PRE-LABORATORY QUESTIONS 1. In this experiment the stock CoCl 2 is CoCl 2 ∙6H 2 O in 2-propanol. Why must contamination with water be avoided? Why is the water originally complexed to the cobalt insignificant? H 2 O can replace the alcohols as a ligand. [Co(H 2 O) 6 ] 2+ is an octahedral complex, water being even smaller than methanol. The stock CoCl 2 concentration is in the millimolar range, so it contributes at most about 10 mM water. Pure water is 55.5 M; 2-propanol is over 10 M and methanol almost 5 M. 10 mM water cannot compete with such high concentrations of alcohols. 2. Crystals of CoCl 2 ∙6H 2 O are pink; crystals of CoCl 2 are blue. What do these colors suggest about the coordination of cobalt in each compound? Pink suggest octahedral coordination and blue suggest tetrahedral coordination.
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