3. What weight of a substance containing no more than 70% MnO2 by mass should be taken for analysis so that after its interaction with 25 ml of 0.1100 N H2C2O4 solution, 5.0 ml of KMnO4 solution is consumed for titration of excess oxalic acid? Take into account when determining the concentration of KMnO4 that the ratio of the equivalent volumes of H2C2O4 and KMnO4 is 1.025.

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