AaßbCcDdEe AaBbCcDdEe Normal No Spacing NameLab section Pre-laboratory Questions

Question

AaßbCcDdEe AaBbCcDdEe Normal No Spacing NameLab section Pre-laboratory Questions: Week 9, Oxidation-Reduction Titrations I: Determination of Oxalate When titrating KMno, against CO, the solution turns purple initially with a few drops of permanganate, then becomes nearly colorless, before eventua becoming pink at the endpoint. Please explain these changes in color based on the chemistry in the lab. (Read through the procedure of the lab) 1. 2. A48.36 ml, aliquot of aqueous KMnO, was used to titrate o.317o g of Na C.0 What is the molarity of the potassium permanganate solution? Make sure you use the balance chemical equation from the lab manual. 3. Why is the KMnO, solution filtered, and why should it not be exposed to light?

Explanation / Answer

1. When titrating KMnO4 with C2O4^2-, the initial purple color is due to addition of purple KMnO4 to the solution. This is localized effect, Mn(+7) state is purple in color. When mixed, the +7 Mn gets reduced to +2 Mn state which is colorless and thus solution goes colorless. After all of C2O4^2- is titrated, any excess of Mn +7 in solution gives pink at the end point due to excess Mn(+7) as KMnO4 in solution,

2. moles of Na2C2O4 taken = 0.3170 g/134 g/mol = 0.0024 moles

moles KMnO4 reacted = 0.0024 moles x 2/5 = 0.00096 moles

molarity of KMnO4 solution = 0.00096 moles/0.04836 L = 0.02 M

3. The solution of KMnO4 is filtereted to remove undissolved impurities or undissolved KMnO4 that might be left in solution.

KMnO4 undergoes phorodecomposition in presence of light to form potassium manganate and manganese dioxide. Therefore, to avoid this photodecomposition, the KMnO4 solution is not exposed to light.

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