Chemistry – Stoichiometry - Relative masses of atoms and molecules | e-Consult
Stoichiometry - Relative masses of atoms and molecules (1 questions)
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Relative molecular mass (Mr) is the mass of a molecule, calculated by summing the relative atomic masses (Ar) of its constituent atoms. It applies to covalent compounds.
Relative formula mass (Mrf) is the mass of a formula unit of an ionic compound, calculated by summing the relative atomic masses (Ar) of the atoms in the formula. It applies to ionic compounds.
Why Mrf is used for ionic compounds: Ionic compounds exist as crystal lattices of ions. Therefore, we use the formula mass to represent the mass of one formula unit (one complete ionic compound) which is the unit of the crystal lattice. We don't deal with individual molecules of ionic compounds; we deal with the formula unit.