Question

1. Why does KMnO4 need to be filtered?

2. Why is KMnO4 usually stored in a glass-stoppered bottle instead of a rubber-stoppered bottle?

3. Why does the solution decolorize on standing after the equivalence point has been reached?

4. If 20.09 mL of 0.0200 M KMnO4 was required to titrate a 0.1750 g sample of K3[Fe(C2O4)3]?3H2O, what is the percent C2O4 2- in the complex?.

Answer 1

1. KMnO4 in neutral aqueous solution is not stable. It slowly gets converted into MnO2. If MnO2 is not filtered out , MnO2 will interfere with all colorimetry expts and also the regular chemical reactions.
2. KMnO4 being a strong oxidizer will interact with rubber and degrade itself.
3. The decoularisation occurs due to the reaction which KMNO4 undergoes. This decoularisation generally produces a manganese salt. Mn+2 is a very light purple, but nowhere near as strongly colored as KMnO4.
4. The balanced equation for permanganate/oxalate is:
   2 MnO4- (aq ) + 5 C2O4-2 (aq ) + 16 H+ (aq ) --> 2 Mn2+ (aq) + 10 CO2 (g ) + 8 H2O (l)
   Two moles of Permanganate will react with 5 moles of Oxalate.
   20.07 mL of 0.0200 M KMnO4 is
   20.07 mL * 0.0200 mol/L * 0.001 L/mL = 0.0004014 moles of KMnO4
   This means that 0.0004014 moles * 5/2 = 0.0010035 moles of Oxalate would be titrated.
   88.01 g/mole (oxalate mol. Wt) * 0.0010035 moles = 0. 08831 grams of Oxalate is present.
   This is 0.08831 grams / 0.1750 grams of complex * 100 = 50.40 %