Pre-lab 2.3.b Weighing by Difference

This experiment begins a series of analytical investigations that will be used to confirm the product composition of your cobalt ammine coordination compound. We begin by determining the percentage composition by mass of cobalt in the coordination compound synthesized in Experiment 2.1. The first step involves quantitative decomposition and reduction of the cobalt(III) complex into the cobalt(II) ion. The decomposition of pentaammine-chlorocobalt(III)chloride [Co(NH3)5Cl]Cl2 is shown as an example in equation (2.6). 

In this experiment, you will prepare a series of [Co(H20)6]2+ complexes of varying concentrations from a standardized stock solution of known concentration. 

By constructing a plot of absorbance vs concentration, you will be able to determine the concentration of cobalt in your sample. 

A stock solution of aqueous cobalt(II), Co2+(aq), of known concentration ( 0.085 m) prepared from C0SO4 7H20-cobalt(II) sulfate heptahydrate will be provided by the instructor. 

Each student must dry his/her sample of cobalt coordination compound in a weighing bottle (top ajar) for about 1 h at 120 °C and cool to room temperature in a desiccator. It is recommended that this be carried out during a pre-lab lecture or at the start of Procedure 2.2.a. 

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