58. a) Why is sodium oxalate a more convenient substance for setting the titer of potassium permanganate than oxalic acid? Why do the first drops of permanganate discolour slowly when oxalic acid (or oxalate) is titrated? What can be done to ensure that the first drops of permanganate discolor quickly?
b) What sample of Na2C2O4 (chemically pure) is required to take to establish the titer of a 0.1 N solution of KMnO4) if a 250 ml volumetric flask and a 20 ml pipette are used?

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