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Chemistry LibreTexts

Concentration of Unreacted Metal Ion

In-class Problem Set #4

1. Calculate the concentration of free calcium(II) in a solution initially prepared with 0.020 M calcium and 0.10 M EDTA4-.

Before beginning this problem set, spend about twenty minutes discussing amino acids and zwitterions, hydrochloride salts, and common ligands such as EDTA and how they function.

Give the students about five minutes to think about what might be occurring and look up Kf values for the complex. Spend about thirty minutes talking about metal-ligand complexes and chelates. Talk about H2O as a ligand and how a hydrated metal ion compares entropically to a metal chelated with EDTA. Briefly discuss the geometry of metal-ligand complexes and how the complexation stoichiometry varies depending on the metal ion and ligand.  With the realization that the Kf value is large, groups are able to set up a table with initial amounts, a line for a complete reaction, and a line for the back reaction.

What is the value of x for the back-reaction based on this Kf expression?

Examine the amount of uncomplexed calcium(II) to confirm that it is quite small.