This is the most common type of stoichiometric problem in high school.

There are four steps involved in solving these problems:

1. Make sure you are working with a properly balanced equation.
2. Using dimensional analysis always starting with the "given."
3. In your molar masses, be sure to carry out your numbers to 2 or 3 decimal places (so that you don't round off too early.)
4. Attend to significant figures at the end of the problem.

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 to moles.

4.Changes moles of reactant to moles of product. The molar ratio (what we call the "molar connections") comes from the coefficients in the balanced equation.

5.Convert moles to grams.

6.Pick up the calculator and hit the following buttons:

(100 x 8 x 64) / (120 x 4) =

The display shows the answer of 106.66666667

7. Round to the correct number of sig figs (which in this case would be 1 SF) and you would report an answer of 100 grams.

• Note that in our dimensional analysis above, since we knew ahead of time that there would only be 1 sig fig in the answer (from the "given"), we didn't need to have an extremely precise molar mass (in other words, not out to several decimal places). However, when you need more than 1 sig fig in your answer, be sure to include several decimal places in your molar masses in the dimensional analysis setup.

Limiting Reactants:

• The reactant that is completely consumed in the reaction.
• It is not present in sufficient quantity to react with all other reactants.
• The reaction stops when it is completely consumed.
• Any remaining reactants are considered "excess reactants".
• The amount of product formed is determined by the "limiting reactant".

An Example:

What mass of water can be produced by 4g H₂ reacting with 16g O₂?
1. Write a balanced equation for the reaction.
2. Solve the problem "twice" -- start first with the 4 grams of hydrogen and determine how much water can be produced; then do the same thing again but this time, start with 16 grams of oxygen and determine how much water can be produced.
3. The least amount of product identifies the limiting reactant.