The mass of FeS2=1000000g×0.05100=500 g.
The number of moles of FeS2=500120=4.167 moles.
2 moles of FeS2 react with 8 moles of NaOH.
Hence, 4.167 moles of FeS2 will react with 82×4.167=16.67 moles of NaOH.
The volume of 6M NaOH solution required is 16.676=2.778 L =2778 ml.