Stoichiometry is the measuring or calculating of the amounts of elements or compounds involved in a chemical change.

Mole is a unit. It is a unit for the amount of element or compound in a chemical reaction. (ex. 2 mols of H2O)

Moles Bridge is a diagram showing the steps for solving stoichiometry questions. Diagram 1 is a Moles Bridge diagram.

Diagram 1:

Terms in a Moles Bridge Diagram

Mass of substance
: The mass of substance usually given in the question. The unit is in grams (g).

Molar mass: Molar mass can be found from the periodic table at the bottom of each element symbol. The unit is in grams (g).

Volume of substance: Volume of substance is usually given in the question. The unit is in litres (L).

22.4 L/mol: 22.4 L/mol is the STP.

# of particles of substance (atoms or molecule): This is usually given in the equation.

Avogadro's number: 6.0221415 × 102

Molar ratio: Molar ratio is a fraction. The numerator will be the number of moles of the unknown substance. The denominator will be the number of moles of the known substance.

Let's look at a few example questions!

1) How many grams of CO2 is produced when reacting with 2.00 mols of O2?

C3H8 + 5 O2 à3 CO2 + 4 H2O

- First, we need to see if the equation is balanced or not. In this question, it is.

- Next, we can start to form our equation. Start with the information given, which is 2.00 mols of O2. Because we start with moles of known substance from the moles bridge, so we need to continue by using the molar ratio. Next, we need to multiply by the molar mass of CO2 to get the g of CO2.

? g CO2 = 2.00 mols O2 (3 mols CO2 / 5 mols O2) (44 g CO2 / 1 mol) = 52.8 g CO2

*How to find the molar mass of CO2? C:12.0 + O: 16.0 × 2 = 44 g CO2
*Be careful of Significant Digits.

2) What is the volume of O2 at STP is if the chemical reaction produces 10.0 L of CO2 at STP? (Use the previous chemical equation.)

3) How many grams of aluminum must be to produced 30.0 grams of aluminum chloride?
2 Al + 6 HCl à2 Al Cl3 + 3 H2

4) Convert the masses of 117.1 g iron (II) oxide into moles.
Fe2O3 + 3 COà2 Fe + 3 CO2

5) How many moles of KClO3 are needed to produce 0.60 moles of oxygen?
KClO3àKCl + O2


2) ? L O2 = 10.0 CO2 (1 mol / 22.4 L) (5 mols O2 / 3 mols CO2) (22.4 L / 1 mol) = 16.7 L O2

3) ? g Al = 30.0 g AlCl3 (1 mol / 133.5 g AlCl3) (2 Al / 2 AlCl3) (27.0 g Al / 1 mol) = 6.07 g Al

4) ? mols Fe2O3 = 117.1 g Fe2O3 ( 1 mol / 159.6 g Fe2O3) = 0.7337 mols Fe2O3

5) 2 KClO3à2 KCl + 3 O2

? g KClO3 = 0.60 mols O2 (2 mols KClO3 / 3 mols O2) = 0.40 mols KClO3