# Definitions

## Relative Isotopic Mass

The relative isotopic mass of an isotope is the mass of an atom of the isotope compared with a mass of exactly 12 for an atom of carbon-12

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## Relative Atomic Mass

The relative atomic mass (Ar ) of an element is the average mass of an atom of the element compared with a mass of exactly 12 for an atom of carbon-12

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## Relative Molecular Mass

The relative molecular mass (Mr) of a substance is the average mass of a molecule of that substance on a scale where the mass of an atom of carbon-12 is exactly 12

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## Relative Formula Mass

The relative formula mass (Mr) of a substance is the average mass of a formula unit of that substance on a scale where the mass of an atom of carbon-12 is exactly 12

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## Mole

A mole is the amount of a substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12

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## Molar Mass

Molar mass is the mass (in g) of 1 mole of a substance. The units of molar mass are g mol-1 (grams per mole)

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## Stoichiometry

Stoichiometry is the relative number of moles of each substance that react

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