Assuming complete ionization, same moles of which of the following compounds will require the least amount of acidified KMnO₄ for complete oxidation

For complete ionization, one mole of FeSO₄ requires 0.5 moles of K₂Cr₂O₇ and one mole of FeSO₃ requires 0.6 moles of K₂Cr₂O₇. One mole of FeC₂O₄ required 0.6 moles of K₂Cr₂O₇. One mole of Fe(NO₂)₂ required 1 mole of K₂Cr₂O₇.

Therefore, FeSO₄ requires least amount of moles of K₂Cr₂O₇.