What is stoichiometry
Chapter 12.1
Vocabulary Word
• stoichiometery: the study of quantitative
relationships btwn the amounts of reactants
and products formed in a chemical rxn.
Stoichiometry
• Chemists need to know how much of each
reactant is needed to optimize a reaction.
• The law of the conservation of mass says
the total mass of the reactants = the total
mass of the products.
• Balanced chemical equations and molar
masses let us calculate the masses of the
reactants and products in a chemical
reaction.
Example
Fe + O2 
Fe2O3
Balanced equation:
4 Fe
+ 3 O2

2 Fe2O3
4 moles of iron + 3 moles oxygen form 2 moles
iron oxide
Example
• 4 moles of iron + 3 moles oxygen form 2
moles iron oxide
• 4 (55.85) + 3(32) = 2 [ 2 (55.85) + 3 (16)] =
• 223.4 + 96 = 2(111.7 + 48)
• 319.4 = 319.4
• the mass of the reactants = the mass of the
products
Vocabulary Word
• mole ratios: ratios btwn # moles of any 2
substances in a balanced chemical equation.
Mole Ratios
2 Al
• ratios:
+ 3 Br2
2 moles Al
3 moles Br2

2 AlBr3
2 moles Al
2 moles AlBr3
3 moles Br2
2 moles AlBr3
• the inverse of each ratio is also true
• 6 ratios can be formed
Mole Ratios
• To determine the number of ratios that can
be formed:
# species (different things) x next lowest #
• 3 species forms 3 x 2 = 6 ratios
• 4 species forms 4 x 3 = 12 ratios
• These ratios allow us to determine the
amount of reactants and to predict the
amount of product formed.