Chemistry » Composition of Substances and Solutions » Determining Empirical and Molecular Formulas

Glossary of Words

Summary of lessons so far

Formula Mass and the Mole Concept

The formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula and is expressed in atomic mass units. The formula mass of a covalent compound is also called the molecular mass. A convenient amount unit for expressing very large numbers of atoms or molecules is the mole.

Experimental measurements have determined the number of entities composing 1 mole of substance to be 6.022 × 1023, a quantity called Avogadro’s number. The mass in grams of 1 mole of substance is its molar mass. Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass (amu) and molar mass (g/mol) for any substance are numerically equivalent (for example, one H2O molecule weighs approximately 18 amu and 1 mole of H2O molecules weighs approximately 18 g).

Determining Empirical and Molecular Formulas

The chemical identity of a substance is defined by the types and relative numbers of atoms composing its fundamental entities (molecules in the case of covalent compounds, ions in the case of ionic compounds). A compound’s percent composition provides the mass percentage of each element in the compound, and it is often experimentally determined and used to derive the compound’s empirical formula. The empirical formula mass of a covalent compound may be compared to the compound’s molecular or molar mass to derive a molecular formula.

Key Equations

• $$\%\text{X} = \frac{\text{mass X}}{\text{mass compound}} × 100\%$$
• $$\frac{\text{molecular or molar mass (amu or }\mathrm{\frac{g}{mol}})}{\text{empirical formula mass (amu or }\mathrm{\frac{g}{mol}})} = n\text{ formula units/molecule}$$
• $$(\mathrm{A_xB_y})_{\text{n}} = \mathrm{A_{nx}B_{ny}}$$

Glossary of words

experimentally determined value of the number of entities comprising 1 mole of substance, equal to 6.022 × 1023 mol−1

Empirical formula mass

sum of average atomic masses for all atoms represented in an empirical formula

Formula mass

sum of the average masses for all atoms represented in a chemical formula; for covalent compounds, this is also the molecular mass

Molar mass

mass in grams of 1 mole of a substance

Mole

amount of substance containing the same number of atoms, molecules, ions, or other entities as the number of atoms in exactly 12 grams of 12C

Percent composition

percentage by mass of the various elements in a compound