The oxidation reaction is as given below.
5SO2+2KMnO4+2H2O→2MnSO4+K2SO4+2H2SO4
Thus, 2 mol KMnO4 = 5 mol SO2.
The molar mass of KMnO4 is 158g/mol .
The number of moles of KMnO4 is 2×1.58 g158 g/mol=0.02 mol.
The number of moles of SO2 required is 0.02 mol×5 mol2 mol=0.05 mol.
2 moles of SO2 will be obtained from one mole of FeS2.
Hence, 0.05 moles of SO2 will be obtained from 0.05 mol2 mol=0.025 mol of FeS2.
The molar mass of FeS2 is 120 g/mol.
Hence, the mass of FeS2 is x=120 g/mol×0.025 mol=3 g.