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Calculating the Number of Moles

The molar mass is the mass of one mole of any substance in grams and is given the symbol M. One mole of carbon-12 has a mass of 12g, so M=12. 1 mole of magnesium-24 atoms has a mass of 24g. This is because a magnesium-24 atom has twice the mass of a carbon-12 atom. A hydrogen atom is 1/12th the mass of a carbon-12 atom. It consists of a single proton (and an electron of negligible mass). The molar mass of hydrogen is 1g.The number of moles of a substance can be found using the equationnumber of moles = Mass / Molar Mass

The relative atomic mass and isotopic mass are given the symbol Ar and the relative formula mass and relative molecular mass are given the symbol Mr.

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Specification: AQA Chemistry (7404), AS-level, 3.1.2.1 Relative atomic mass and relative molecular mass. Topic: Amount of Substance.

Key terms in this lesson

Relative atomic mass
The weighted mean mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.
Relative molecular mass
The weighted mean mass of a molecule compared to 1/12th the mass of a carbon-12 atom.
Relative formula mass
The mass of a formula unit compared to 1/12th the mass of a carbon-12 atom.
Mole
The amount of substance that contains as many particles as there are atoms in exactly 12g of carbon-12.

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