What's the difference between molecular and formula mass?

Molecular mass refers to the mass of a single molecule, while formula mass refers to the mass of a formula unit in ionic compounds.

Molecular mass is a term used in chemistry to represent the mass of one molecule of a substance. It is calculated by adding up the atomic masses of all the atoms present in a molecule. For example, the molecular mass of water (H2O) is calculated by adding the atomic masses of two hydrogen atoms and one oxygen atom. This is typically used for covalent compounds, where atoms share electrons to form molecules.

On the other hand, formula mass is a term used to describe the mass of one formula unit of an ionic compound. Ionic compounds are formed by the electrostatic attraction between positive and negative ions, and they do not form discrete molecules like covalent compounds. Instead, they exist as a repeating pattern of ions, known as a crystal lattice. The formula mass is calculated by adding up the atomic masses of all the atoms in the empirical formula of the compound. For example, the formula mass of sodium chloride (NaCl) is calculated by adding the atomic masses of one sodium atom and one chlorine atom.

In summary, the key difference between molecular mass and formula mass lies in the type of compound they refer to. Molecular mass is used for covalent compounds, which consist of discrete molecules, while formula mass is used for ionic compounds, which consist of a repeating pattern of ions. Both terms, however, involve the calculation of the sum of the atomic masses of all the atoms present in the molecule or formula unit.