31. Calculate the mass fraction (%) of MnO2 in natural pyrolusite, if a sample weighing 0.4000 g was treated with dilute H2SO4 containing 0.6000 g of H2C2O4 * 2H2O and the excess of oxalic acid was titrated with 23.26 ml of 0.1129 N. KMnO4 (feq=1/5).
Answer: 74.91%.

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