The atom economy, the mass of the wanted product as a percentage of the mass of all the reactants in the balanced equation, is fixed by the equation: making iron by Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂ has 46 percent, making ammonia has 100.
Iron is 2 × 55.845 = 111.7 g of the 159.7 + 3 × 28.01 = 243.7 g of reactants, 45.8 percent, because the carbon dioxide carries off the rest. Ammonia is the whole of the mass of nitrogen and hydrogen, 100 percent. Unlike the yield it cannot be improved by working harder: only by choosing a different reaction.
It counts every product that is not the wanted one as waste, even when it is useful (the carbon dioxide may be sold, the other product of a reaction may be the real aim), and it ignores solvents, catalysts and energy. It is a first screen of a reaction’s efficiency, not the whole of its environmental cost.
Estimate with the rule, then check it against the calculator that models it properly.
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