Introduction to Stoichiometry

Stoichiometry is the division of chemistry that looks at the relationships between the amounts of reactants and/or products. In Greek, stoikhein means element and metron means measure, so stoichiometry literally translated means the measure of elements.

Stoichiometry is all about determining the correct proportions to combine reactants in. This might sound scary (stoichiometry is certainly a mouthful!) but if you have ever cooked or baked you have already done stoichiometry.

How much flour would we need if we wanted to make 2 dozen cookies?

If you answered 4 ½ cups flour then you are a chemical stoichiometer!

How much flour would we need if we wanted to make ½ dozen cookies?

2 ⅛ cups flour

Given the chemical equation above, how many of each reactant would we need to make 6 water molecules?

3 CH₄ and 6 O₂

because we can triple the recipe to make 6 water

3CH₄ + 6O₂ → 3CO₂ + 6H₂O

Chemists do not usually conduct reactions with single molecules because they are too small to work with in the lab.

Instead, chemists work with moles of a chemical because it is a large enough amount that it can be converted into grams and weighed in the lab. We can interpret the coefficients in a chemical equation to stand for moles.

To find out how much "one mole methane molecules" weighs in the lab (or any chemical for that matter) we multiply the number of moles by the molar mass.

1 mol x (12.01 + 4(1.01)) g/mol = 16.05 g

So in the lab one mole of methane will weigh 16.05g.

Different from single molecules we can have half a mole.

So we can make 5 moles of water using the above reaction:

it requires 2.5 moles CH₄ and 5 moles O₂

because we can multiply the recipe by 2.5

2.5CH₄ + 3O₂ → 2.5CO₂ + 5H₂O