Relative Atomic Mass and Relative Molecular (Formula) Mass

The relative atomic mass (Ar) refers to the weighted mean mass of an atom of an element compared to 1/12 of the mass of an atom of carbon-12. It considers the different isotopes of an element and the abundance of each isotope.
The relative molecular mass (Mr) is the weighted mean mass of a molecule compared to 1/12 of the mass of an atom of carbon-12.
The relative formula mass is a term used in the same way as relative molecular mass, but it’s used for ionic compounds as they don’t have molecules.
Calculating relative formula mass is a vital skill and can be done by adding up the relative atomic masses of all atoms present in a formula.

For instance, water (H2O) has a relative formula mass of (2*1) + 16 = 18 (Hydrogen’s atomic mass is 1 and there are two atoms, and the atomic mass of oxygen is 16)

Mole is the unit of measurement in chemistry. One mole of any substance contains the same number of entities (atoms, molecules, ions etc.) which is approximately 6.02 x 10^23, also known as Avogadro’s number.
Avogadro’s number is incredibly vital; remember that one mole of atoms of an element will have a mass in grammes equal to the relative atomic mass (Ar) for that element. Similarly, one mole of molecules of any substance will have a mass in grammes equal to the relative molecular mass (Mr) for that substance.
To find the number of moles in a substance, the equation can be used, n = m/M where ‘n’ is number of moles, ‘m’ is mass in grammes, and ‘M’ is Mr (relative molecular mass) or Ar (relative atomic mass) of the substance.
Another critical concept is percentage composition, which shows the mass of each element as a percentage of the total mass of a compound. This can be used to find empirical formulas.