78. Solving a chemistry problem

7. A. Is it possible to reduce MnO4- to Mn2 by the action of Fe2 in an aqueous solution if its pH = 3, and the concentrations of other ions participating in the redox reaction are equal to 1 mol / dm3?
B. Determine the mass fraction of calcium carbonate in limestone, if calcium was isolated from a limestone weighing 0.3864 g in the form of calcium oxalate, which was dissolved in sulfuric acid and titrated with 45.10 cm3 of KMn04 solution with a titer of CaCO3 equal to 0.007390 g/cm3 .