The subject of stoichiometry involves quantitative calculations based on chemical formulas and chemical equations.
3.1 Molecular Masses and Formula Masses—Molecular masses and formula masses are the masses, expressed in atomic mass units (u), of individual molecules and formula units. They are calculated from the masses of the atoms represented in the molecular or empirical formulas, respectively. Molecular mass applies only to molecular compounds; formula mass is appropriate for ionic compounds.
3.2 The Mole and Avogadro’s Number—A mole (mol) is an amount of substance containing a number of elementary entities (atoms, molecules, formula units, etc.) equal to the number of atoms in exactly 12 g of carbon-12. This number is Avogadro’s number,NA = 6.022 × 1023 mol-1. The mole is the SI unit for the amount of a substance and is used extensively in chemical equations and calculations.
3.3 The Mole and Molar Mass—The mass, in grams, of one mole of substance is called the molar mass; it is numerically equal to an atomic, molecular, or formula mass but carries the unit g/mol. Conversions between the number of moles and the number of grams of a substance require molar mass as a conversion factor. Calculations involving volume, density, and numbers of atoms or molecules can also be used to determine molar quantities.
3.4 Mass Percent Composition from Chemical Formulas—The mass percentages of individual elements in a compound can be determined from the chemical formula and molar mass with the following equation:
The collection of these mass percentages represents the mass percent composition of the compound.
3.5 Chemical Formulas from Mass Percent Composition—An empirical formula can be established from the mass percent composition of a compound by calculating molar ratios of the different elements present in the compound. Empirical formulas calculated in this way may or may not be equivalent to the molecular formula. To establish a molecular formula, we must also know the molecular mass.
3.6 Elemental Analysis: Experimental Determination of Mass Percent Composition—The mass percents of carbon, hydrogen, and oxygen in organic compounds can be determined by combustion analysis. Other methods are required to determine the mass percent composition of inorganic compounds.
3.7 Writing and Balancing Chemical Equations—A chemical equation uses symbols and formulas for the elements and/or compounds involved in a reaction. A chemical equation portrays the progress of a reaction, indicated by an arrow, from reactants toproducts. Stoichiometric coefficients are placed before the symbols or formulas in the equation to balance the equation. As required by the law of conservation of mass, for each element in a balanced chemical equation the number of atoms on the product side of the equation will be the same as on the reactant side.
3.8 Reaction Stoichiometry—Stoichiometry involves quantitative relationships in a chemical reaction. Stoichiometric factors—also called mole ratios—are based on the coefficients in the balanced equation and are used to relate moles of one reactant or product to another. Molar masses and stoichiometric factors, together with other factors, are used to determine information about one reactant or product in a chemical reaction from known information about another. The strategy for reaction stoichiometry calculations can be outlined diagrammatically, as suggested below.
3.9 Limiting Reactants—The limiting reactant is the reactant that is completely consumed in a reaction. Its quantity determines the theoretical quantity of the products formed. Other reactants are said to be present in excess. In some stoichiometry problems the limiting reactant must first be identified through a preliminary calculation.
3.10 Yields of Chemical Reactions—The calculated quantity of a product is thetheoretical yield of a reaction. The quantity physically obtained from a chemical reaction, called the actual yield, is often less and is commonly expressed as a percentage of the theoretical yield.
3.11 Solutions and Solution Stoichiometry—Solutions are formed by dissolving one substance, the solute, into another substance, the solvent. The solute is usually the component present in a lesser amount. The molarity (M), or molar concentration, of a solution is the number of moles of solute per liter of solution.
Common calculations involving solutions include relating an amount of solute to solution volume and molarity. Solutions of a desired concentration are often prepared from more concentrated solutions by dilution. Dilution increases the volume of a solution, but the amount of solute is unchanged. As a consequence, the concentration decreases. Stoichiometric calculations for reactions in solution often use molarity or its inverse as conversion factors, in addition to the stoichiometric factor and possibly other conversion factors.