# Amount of Substance

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## Relative masses, the Avogadro constant and moles

Relative atomic mass

Carbon-12 is used as a baseline for relative atomic mass. One twelfth of the relative atomic mass of carbon-12 is given a value of exactly 1:

The relative atomic mass is the weighted average mass of an atom of an element, taking into account its naturally occurring isotopes, relative to 1/12th the relative atomic mass of an atom of carbon-12.

Relative atomic mass = average mass of one atom of an element/a twelfth of the mass of an atom of carbon-12

Therefore:

Relative atomic mass = (average mass of one atom of an element*12)/mass of an atom of carbon-12

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## Relative masses, the Avogadro constant and moles

Relative molecular mass

The relative molecular mass of a molecule is the mass of that molecule compared to a twelfth of the relative atomic mass of an atom of carbon-12.

The relative molecular mass can be found by adding up the relative atomic masses of all the atoms present in the molecule and this can be found from the formula.

For example:

methane CH4 Relative atomic mass:12.0+(4*1.0)

Molecular mass: 16.0

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## Relative masses, the Avogadro constant and moles

The number of atoms in 12g of carbon-12.

A helium atom is 4 times heavier than a hydrogen atom. A lithium atom is 7 times heavier than a hydrogen atom. In order to get the same number of atoms in a sample of helium or lithium, as the number of atoms in 1 gram of hydrogen, you must take 4grams of helium or 7 grams of lithium.

This same logic applies to molecules.

Water (H2O) has as relative molecular mass of 18. So one molecule of water is 18 times heavier than one atom of hydrogen. Therefore, 18g of water contains the same number of molecules are there are atoms in 1g of hydrogen.

Avogadro constant is written as: 6.022 * 10^23

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## Relative masses, the Avogadro constant and moles

The mole

The amount of substance that contains 6.022 * 10^23 (Avogadros constant) particles is called a mole.

The relative atomic mass of any element in grams contains one mole of atoms.

The relative molecular mass (or relative formula mass) of a substance in grams contains one mole of entities.

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## Relative masses, the Avogadro constant and moles

Number of moles

In order to find out how many moles are present in a particular mass of a substance, the substance's formula has to be known.

Number of moles = mass in grams/mass of 1 mole in g

Example:

How many moles of atoms are there in 64.2g of sulfur, S?

The relative atomic mass of sulfur is 32.1, so 1 mole of sulfur has a mass of 32.1g.

Therefore, the number of moles = 64.2/32.1 = 2.00mol

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