Stoichiometry: Chemical Formulas and Equations

Quantitative information from balanced equations

The coefficients in a balanced chemical equation can be interpreted both as the relative numbers of molecules involved in the reaction and as the relative number of moles.

For example, in the balanced equation:

the production of two moles of water would require the consumption of 2 moles of H₂ and one mole of O₂. Therefore, when considering this particular reaction

would be considered to be stoichiometrically equivalent quantitites. Represented as:

2 mol H₂ 1 mol O₂ 2 mol H₂O

Where '' means "stoichiometrically equivalent to".

These stoichiometric relationships, derived from balanced equations, can be used to determine expected amounts of products given amounts of reactants. For example, how many moles of H₂O would be produced from 1.57 moles of O₂ (assuming the hydrogen gas is not a limiting reactant)?

The ratio is the stoichiometric relationship between H₂O and O₂ from the balanced equation for this reaction.

For the combustion of butane (C₄H₁₀) the balanced equation is:

Calculate the mass of CO₂ that is produced in burning 1.00 gram of C₄H₁₀.

First of all we need to calculate how many moles of butane we have in a 100 gram sample:

now, the stoichiometric relationship between C₄H₁₀ and CO₂ is: , therefore:

The question called for the determination of the mass of CO₂ produced, thus we have to convert moles of CO₂ into grams (by using the molecular weight of CO₂):

Thus, the overall sequence of steps to solve this problem were:

In a similar way we could determine the mass of water produced, or oxygen consumed, etc.

1996 Michael Blaber