It is easy. 1 Fe oxidizes by 1 ON up, 2 N reduces by 1 ON down each. so 1 O oxidizes 1 ON up. It directly leads to 4 Fe, 8 N and 1 O2, i.e 4 Fe(NO3)2 -> 2 Fe2O3 + 8 NO2 + O2. I remember solving such things in 1980, before entering high school.

Be aware that both reactions are sums of two independent reactions (oxidation of iron(II) and oxidation of oxalate) and you can theoretically formulate infinite number of different reaction equations, depending on the chosen iron : oxalate ratio/

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The shielding effect is about the net force, as the vector sum of all force contributions between particular charges. One can quite well apply classical electrostatic analogy.

After 14 years, "chemically pure" is just a label.

Quote essential parts or summaries of links you provide, as questions should be self-contained even in cases the links stop working. Also, users should be able to evaluate the question even without following the links that should serve just for more details or reference.

It is unclear what resources you have consulted before asking this question and what conclusion you have made from that.

Different ingredients and their ratio with different procedures lead to different results. If you want details, you need to ask producers.