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6.S: Quantitative Relationships in Chemistry (Summary)

• Stoichiometry is the relationship between the masses of chemical reactants and products in a given chemical reaction. The coefficients placed in a chemical equation in order to balance it are called the stoichiometric coefficients. Molar stoichiometry is simply the expression of the coefficients of a reaction in terms of moles of reactants and products.
• In order to find the number of moles of a product that is produced in a chemical reaction when you are given moles of reactant, simply multiply the moles of reactant by the stoichiometric ratio relating that reactant and the desired product; i.e., $(molreactant)\times \left ( \frac{molproduct}{molreactant} \right )$
• In order to find the number of moles of a product that are produced in a chemical reaction when you are given mass of reactant, simply divide the mass of reactant by the molar mass (to get moles reactant) and then multiply by the stoichiometric ratio relating that reactant and the desired product; i.e., $\left ( \frac{grams}{grams/mol} \right )\times \left ( \frac{molproduct}{molreactant} \right )$
• Always remember, mass divided by molar mass equals moles; $\left ( \frac{grams}{grams/mol} \right )=mol$
• The mass or the number of moles that you calculate for a product based on reaction stoichiometry is called the theoretical yield for the reaction. The amount of material that you actually isolate from a given reaction is called the actual yield and it is always less than the theoretical yield. The ratio of the actual and theoretical yields, expressed as a percentage is called the percentage yield.
• If a reaction requires more than one reactant and if you are given the mass, or the number of moles of each reactant, you must approach the calculation as a limiting reactant problem. To solve a limiting reactant problem, simply perform the standard mass calculation for each reactant, noting the mass (or number of moles) of product formed in each calculation. The reactant that yields the smallest amount or product from these calculations is called the limiting reactant. Reactants that yield larger amounts of products in these calculations are called excess reactants. The theoretical yield in the reaction will be based solely on the calculated amount for the limiting reactant.
• If a reactant in a chemical reaction is said to be “in excess”, you assume that you have unlimited amount of the reactant, and that it will never be the limiting reactant.