Relative Masses (OCR GCSE Combined Science A (Gateway)): Revision Note

Relative Masses

• Relative atomic mass has the symbol Ar

• This is the average mass of the atoms of an element measured relative to the mass of one atom of carbon-12 

• This is calculated from the mass number and relative abundances of all the isotopes of a particular element

• Relative molecular mass is the average mass of the atoms of one molecule of an element or compound measured relative to the mass of one atom of carbon-12 

• Relative formula mass has the symbol Mr and it refers to the total mass of the molecule

• To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

Calculating Relative Formula Mass

• In accordance with the Law of Conservation of Mass, in a balanced chemical equation, the sum of the relative molecular masses of the reactants will be the same as the sum of the relative molecular masses of the products