Relative Atomic Mass & Relative Molecular Mass (AQA A-Level Chemistry): Revision Notes
1.2.1 Relative Atomic Mass & Relative Molecular Mass
Understanding relative masses is essential for calculating the quantities involved in chemical reactions, particularly for determining the mass of atoms, molecules, or compounds.
Relative Atomic Mass
The relative atomic mass () of an element is the average mass of an atom of that element compared to 1/12th the mass of a carbon-12 atom. This value is dimensionless, meaning it has no unit.
Key points:
- Carbon-12 is chosen as the reference because it is non-toxic, solid at room temperature, and easily separable.
- values account for the natural isotopic distribution of the element.
Example: If an element has an of 24, this means an average atom of that element is 24 times heavier than 1/12th the mass of a carbon-12 atom.
Relative Molecular Mass
The relative molecular mass () is the average mass of a molecule compared to 1/12th the mass of a carbon-12 atom. Like , is dimensionless.
- For covalent compounds, is used to describe the sum of the relative atomic masses of the atoms within a molecule.
Example: For water
Relative Formula Mass (RFM)
The relative formula mass (RFM) is used for ionic compounds, but is often accepted for both ionic and covalent compounds. It is calculated by summing the relative atomic masses of all the ions in the formula unit.
Example: For sodium chloride (NaCl):
Molar Mass
The molar mass is the mass of one mole of a substance, typically measured in grammes per mole (g/mol). While and are dimensionless, molar mass links these relative values to the physical mass of a substance.
Example: The molar mass of water is 18 g/mol, meaning one mole of water molecules weighs 18 grammes.
Key Equations
- or = Relative Mass
- Molar Mass (g/mol) = Relative Mass ( or ) in grammes per mole