Stoichiometry is the quantitative study of chemical changes.

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

Steps in solving a mass-mass problem:

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 125 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.

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 dimensional analysis 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.

Mass-Mass Problems

 
Copper and Silver Metals

 
More practice problems

 
Stoichiometry

The limiting reactant is the reactant that is completely consumed in the reaction.

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

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.

The problem solution:

1. Write a balanced equation for the reaction.

2. Convert both reactant quantities to moles.

• 16 grams of oxygen cannot produce as much water as 4 grams of hydrogen. In other words, 16 grams of oxygen will be used up in the reaction before 4 grams of hydrogen.
   
• Oxygen is the "limiting" reactant.
   
• Use oxygen for the calculation of product amount.

The theoretical yield for this problem is 18 grams. If you performed this reaction in the lab, your actual yield might be less. Can you think of reasons why?

Limiting Reactant Problems

 
Percent Yield

• The quantity of product that is calculated to form when all the limiting reactant is used up is called the theoretical yield.
   
• The amount of product actually obtained in a reaction is called the actual yield.
   
• The actual yield is almost always less than (and never greater than) the theoretical yield.

Sample problem:

Given the reaction:

A. If you start with 155 g of Fe₂O₃ as the limiting reactant, what is the theoretical yield of Fe?

B. If the actual yield of Fe was 87.9 g, what was the percent yield?

 
A Little Bit Of Everything Stoichiometry