Relative Masses (OCR Gateway GCSE Chemistry: Combined Science)

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3 March 2022

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

The relative atomic mass of every element is given on the Periodic Table. It is the larger of the two numbers.

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

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