Definition

Noun: - (Chemistry): A dimensionless quantity representing the ratio of the average mass of one molecule of a substance to one-twelfth of the mass of an atom of carbon-12. It is numerically equal to the sum of the relative atomic masses of all the atoms in a given molecular formula.

Usage

• Relative molecular mass is a fundamental concept in chemistry used to quantify the mass of molecules. It is crucial for stoichiometric calculations, such as determining empirical and molecular formulas, and for converting between the mass of a substance and the amount of substance (moles).
• The term is often used interchangeably with molecular weight, though strictly speaking, molecular weight is a weighted average mass that accounts for isotopic distribution, while relative molecular mass is calculated from a specific molecular formula using standard atomic weights.

Examples

• The relative molecular mass of water (H₂O) is approximately 18.015, calculated as (2 × 1.008) + 15.999.
• To find the relative molecular mass of glucose (C₆H₁₂O₆), you sum the relative atomic masses of six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.
• Knowing the relative molecular mass is essential for preparing solutions with a specific molar concentration.

Advanced Usage

• "Relative molecular mass" is sometimes abbreviated as Mᵣ.
• In polymer chemistry, the concept is extended to relative molar mass, which refers to the average mass of polymer chains, as polymers are not single molecules of a fixed size.

Variants and Related Words

• Molecular Weight (n): A closely related term often used synonymously in practice. It is technically the average mass of a molecule, calculated using the natural abundance of isotopes.
• Formula Mass (n): Used for ionic compounds (like NaCl) which do not exist as discrete molecules. It is calculated in the same way—by summing the relative atomic masses of the atoms in the formula unit.
• Molar Mass (n): The mass of one mole of a substance, expressed in grams per mole (g/mol). Its numerical value is identical to the relative molecular (or formula) mass.

Synonyms

• Molecular Weight
• Formula Weight (for ionic compounds)

Notes on Meaning

• The key aspect of relative molecular mass is that it is a , dimensionless number. It compares the mass of a molecule to a standard (¹²C). It does not have units like grams.
• Its value is obtained by simple addition from the molecular formula and a table of standard relative atomic masses.

Noun

1. (chemistry) the sum of the relative atomic masses of the constituent atoms of a molecule