Key concepts in chemistry Study Guide

In a chemical reaction, the amounts of reactants combine in fixed proportions according to the balanced equation (their stoichiometric ratios). If one reactant is entirely used up while others remain, the reaction cannot proceed further, as there is no more of that specific substance to react with the excess. This means the reactant that is not in excess—called the limiting reactant—determines the maximum possible amount of product formed. The excess reactants do not affect the final yield, since no more product can form after the limiting reactant is consumed.

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. By using the masses of reactants and products, we can deduce the stoichiometric coefficients in a balanced chemical equation. The key steps involve converting the given masses into moles using the molar mass of each substance. Once the moles are known, dividing each by the smallest number of moles present gives a mole ratio. This ratio, when simplified to whole numbers, represents the stoichiometric coefficients. For example, if a reaction produced 0.1 mol of product A for every 0.2 mol of product B, the mole ratio would be 1:2. This method relies on the law of conservation of mass, ensuring that the total mass of reactants equals the total mass of products.