Relative Atomic Mass and Relative Molecular (Formula) Mass

The relative atomic mass (Ar) of an element refers to the average mass of one atom of an element relative to one twelfth of the mass of a carbon-12 atom. It takes into account the different isotopes of the element and their relative abundances.
To find the relative atomic mass of an element from its isotopic composition, multiply the mass number of each isotope by the percentage abundance. Then, add these calculations together to get a total. Finally, divide the total by 100 to get the relative atomic mass.
The relative molecular mass, also known as the relative formula mass (Mr), is a similar concept to relative atomic mass but applies to molecules and formula units. This value is the sum of the relative atomic masses of all atoms in a molecule or formula unit.
The relative molecular mass of a compound or molecule can be calculated by adding up the relative atomic masses of all its constituent atoms. For example, for water (H2O), the relative formula mass is calculated as (2 x hydrogen) + oxygen = (2x1) + 16 = 18.
These concepts are crucial in stoicheiometry, the calculation of quantities in chemical reactions, where relative atomic and molecular masses are used to determine molecular and empirical formulas, and to solve problems related to reactants and products in chemical reactions.
The unit for relative atomic mass and relative molecular mass is the atomic mass unit (amu), although no unit is usually written because these are relative values.
It’s important to remember that the relative atomic and molecular masses are dimensionless quantities. This means they have no physical units of measurement associated with them. They simply give an indication of how massive an atom or molecule is compared to another.