Relative Atomic Mass and Relative Molecular (Formula) Mass

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.