Iron can be obtained by reduction of iron oxide (Fe₃O₄) with CO according to the reaction:
Fe₃O₄ + 4CO → 3Fe + 4CO₂
How many kilograms of Fe₃O₄ should be heated with CO to get 3 kg of iron?

$\underset{232 \mathrm{g}}{{\mathrm{Fe}}_3{\mathrm{O}}_4}+4\mathrm{CO}\to \underset{56\times 3 =168 \mathrm{g}}{3\mathrm{Fe}}+4{\mathrm{CO}}_2$
3 moles of Fe is produced from 1 mole of Fe₃O₄
168 g of Fe is produced from 232 g of Fe₃O₄
3 kg of Fe will be produced from $\frac{232}{168}\times 3000 \mathrm{g}$
= 4142.8 g or 4.14 kg of Fe₃O₄