Stoichiometry

Stoichiometry deals with the numerical relationship between compounds in a balanced chemical reaction by following the Law of Conservation of Mass. This relationship is depicted through the use of coefficients, which tells us the number of moles for each compound. 

Coefficients-Balanced-Reaction-Mole-Map-RatioBalanced Equation

The given equation would mean 3 moles of H2 reacts with 1 mole of N2 to produce 2 moles of NH3.

The Limiting Reactant

Within stoichiometry, the limiting reactant or limiting reagent represents the compound that is totally consumed when a chemical reaction occurs. The maximum amount of product formed from the limiting reactant called the theoretical yield. 

In order to determine the theoretical yield of a reaction you must utilize the stoichiometric chart. 

Stoichiometric-Chart-Avogadro-NumberStoichiometric ChartStoichiometry 

In order to calculate the theoretical yield of a chemical reaction we must deal with stoichiometry and have a balanced equation. 

First, always make sure to check to see if your chemical equation is balanced.  

Unbalanced-Chemical-EquationUnbalanced Chemical Equation Now let's take on a typical question dealing with stoichiometry and given quantities for the reactants. 

If 115.0 g Fe reacts with 72.3 g O­2 to produce Fe2O3, what is the amount of product (in grams) formed? 

STEP 1: Balance your chemical equation if needed. 

Balanced-Chemical-Equation-IntegersBalanced Chemical Equation

STEP 2: Find the atomic masses of the elements from the periodic table

Atomic-Masses-Periodic-TableAtomic Masses from Periodic Table

Now find the molar masses of Fe, O­2 and Fe2O3

Molar MassesMolar Masses of Molecular Formulas

STEP 3: Convert the grams of reactants into moles. 

Grams-to-moles-reactantsGrams to moles conversion (Reactants)

STEP 3: Convert the moles of reactants by doing mole-to-mole comparisons. 

Balanced-Chemical-EquationBalanced Chemical Equation

During this step we use the coefficients from the balanced equation. 

Mole-mole-comparisonsMole to Mole comparisons

STEP 4: Convert the moles of product into grams of product. 

Moles-to-grams-conversionMoles to grams conversion (products)

STEP 5: Determine the theoretical yield. 

The theoretical yield, sometimes referred to as the 100% yield, represents the smaller amount. The theoretical yield is 165 g Fe2O3

STEP 5: Determine the limiting reactant and the excess reactant. 

The limiting reactant is the compound that gives the smaller amount of product, while the excess reactant is the compound that gives the larger amount of product. Therefore Fe is the limiting reactant and O­2 is the excess reactant. 

Beyond Stoichiometry

Stoichiometry is one of the most fundamental calculations you’ll do in chemistry. It shares connection with other concepts such as mass conversionsempirical formulapercent yieldmolarity and solution chemistry