Understanding Stoichiometry and the Mole Concept

• Stoichiometry is the study of quantitative relationships or ratios between two or more substances when undergoing a physical change or chemical reaction.
• It is based on the law of conservation of mass, which states that the total mass of the reactants equals the total mass of the products.
• Stoichiometry is a tool for predicting the amounts of reactants and products in a chemical reaction.

Mole Concept

• The mole (symbol: mol) is the base unit of amount of substance in the International System of Units (SI).
• It is defined as exactly 6.02214076 × 10^23 elementary entities.
• Avogadro's number (Na) defines this constant.
• The elementary entities can be atoms, molecules, ions, or other specified particles.
• The mole is used to conveniently express amounts of reactants and products in chemical reactions.
• The molar mass of a substance is the mass of one mole of that substance, expressed in grams per mole (g/mol).
• The molar mass of an element is numerically equal to its atomic weight in atomic mass units (amu).
• The molar mass of a compound is numerically equal to its formula weight in amu.
• The number of moles (n) of a substance can be calculated using the formula: n = mass / molar mass.

Reactant-Product Relationships

• A balanced chemical equation provides the mole ratio between reactants and products.
• The coefficients in a balanced equation represent the relative number of moles of each substance involved in the reaction.
• For example, in the reaction 2H2 + O2 → 2H2O, two moles of hydrogen react with one mole of oxygen to produce two moles of water.
• This mole ratio can be used to calculate the amount of product formed from a given amount of reactant, or vice versa.
• To perform stoichiometric calculations:
• Write the balanced chemical equation for the reaction.
• Convert the given amounts of reactants or products into moles.
• Use the mole ratio from the balanced equation to calculate the moles of the desired product or reactant.
• Convert the moles of the desired substance back into mass or other units as required.

Limiting Reactants

• In many reactions, one reactant is completely consumed before the others.
• The reactant that is completely consumed is called the limiting reactant.
• The limiting reactant determines the maximum amount of product that can be formed in the reaction.
• The other reactants are present in excess.
• To determine the limiting reactant:
• Calculate the number of moles of each reactant.
• Divide the number of moles of each reactant by its stoichiometric coefficient in the balanced equation.
• The reactant with the smallest value is the limiting reactant.
• Once the limiting reactant is identified, the amount of product formed can be calculated based on the amount of the limiting reactant.
• The theoretical yield is the amount of product calculated based on the complete reaction of the limiting reactant.
• The actual yield is the amount of product actually obtained from the reaction.
• The percent yield is the ratio of the actual yield to the theoretical yield, expressed as a percentage: percent yield = (actual yield / theoretical yield) x 100%.
• Reactions do not always proceed to completion or may have side reactions, leading to actual yields that are less than the theoretical yields.