The most common type of stoichiometry calculation is a mass-mass problem.
The question looks like this: "given this amount of reactant, how much product will form?"

1. Write a balanced equation for the reaction.
2. Write the given mass on a factor-label form.
3. Convert mass of reactant to moles of reactant.
4. Convert moles of reactant to moles of product.
5. Convert moles of product to grams of product.
6. Pick up the calculator and do the math.

Mass-Mass Sample Problem: If iron pyrite, FeS2, is not removed from coal, oxygen from the air will combine with both the iron and the sulfur as coal burns. If a furnace burns an amount of coal containing 100 g of FeS2, how much SO2 (an air pollutant) is produced? 1. Write a balanced equation showing the formation of iron (III) oxide and sulfur dioxide. 4FeS2 + 11O2 2 Fe2O3 + 8 SO2 2. Write the mass information given in the problem. 3. Convert grams of FeS2 to moles of FeS2. 4. Changes moles of FeS2 (reactant) to moles of SO2 (product). This ratio comes from the coefficients in the balanced equation. Notice that the ratio was reduced from 8 : 4 to 2 : 1 when placed in the factor-label form. While reducing is not absolutely necessary (the ratio will cancel properly even if not reduced), a good chemistry student notices such things and will do it. 5. Convert moles of SO2 to grams of SO2 . 6. All units have been canceled except for grams of SO2 (product). The problem has been solved. Pick up the calculator and do the math. 107 grams of SO2 will be produced.

1. Write a balanced equation for the reaction.
2. Convert both reactant quantities to moles.
3. Determine the moles of product that could be formed by each reactant.
4. The least amount in step #3 identifies the limiting reactant.
5. Use that number of moles of product to determine the mass produced.