Formula Mass and Mole Concepts of Compounds

The smallest representative unit of a compound is the molecule (or the formula unit), depicted by a chemical formula.

The atomic masses of all atoms in the chemical formula, if added, give the molecular mass (or formula mass) of the compound, measured in atomic-mass-unit.

For example, to find the molecular mass of sucrose or table sugar, the atomic mass of each element is multiplied with the number of atoms of the corresponding element. The sum-total gives the molecular mass of sucrose.

The mass of all molecules (or formula units) present in one mole of a chemical compound is called its molar mass, measured in g/mol.

The molar mass of a compound is numerically equal to its molecular mass (or formula mass). For example, sucrose, with molecular mass of 342.3 amu, will have a molar mass of 342.3 g/mol.

Knowing the actual number of molecules or particles present in a chemical substance is fundamental to understanding its composition, interactions, and combinations.

However, in a given sample of a compound, counting the actual number of molecules by ordinary means is impractical. By employing the relationship between molar mass, mole, and Avogadro’s number, the total molecules present can be calculated.

According to the concept, the molar mass of a compound is equivalent to one mole of the compound, constituting 6.022 × 10²³ particles. Therefore, 342.3 grams of sucrose contain 6.022 × 10²³ sucrose molecules.

However, if a sucrose sample weighs 150 grams for instance, then the number of molecules can be determined by calculation using a cross-multiplication method. The result shows that 150 grams of sucrose constitute 2.63 × 10²³ sucrose molecules.

The percentage of each element in a compound can be expressed using a mass-percent value. To compute, the mass of the element in one mole of the compound is divided by the total mass of one mole of the compound and converted to a percentage.

In practice, to calculate the mass percent of hydrogen in sucrose, the total mass of hydrogen atoms and the total mass of sucrose molecules is included in the formula. The computed value shows that sucrose contains 6.4 mass-percent hydrogen.