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Discovering chemistry
Discovering chemistry

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3 Introducing the mole

The mole is a measure of amount of substance.

Before exploring what this means this in more detail, recall what you learned about ‘weighing’ atoms in session 1.

The relative atomic mass (RAM) of an element is the average mass of an atom of the element relative to one-twelfth the mass of an isotope of carbon-12.

This leads you on to the concept of relative molecular mass (RMM) which is simply the sum of the relative atomic masses of the atoms in a molecule. As this term can only be strictly applied to molecules, you will often find the term relative formula mass (RFM) used instead, which covers ionic substances like sodium chloride (NaCl) and calcium oxide (CaO).

  • Calculate the relative molecular mass of water, given the relative atomic masses of hydrogen and oxygen are, 1.01 and 16.0 respectively.

  • RMM (H2O) = [2 x RAM(H)] + RAM (O)]

    = (2 x 1.01) + 16.0

    = 18.02.

  • What is the relative formula mass of sodium chloride (NaCl), given the relative atomic masses of sodium and chlorine are 23.0 and 35.5 respectively.

  • RFM (NaCl)= (23.0 + 35.5) = 58.5.

Now back to the mole.

One mole of an element or compound has a mass equal to its relative atomic mass, its relative molecular mass or its relative formula mass expressed in grams.

Also, the number of formula units (which may be atoms, molecules, groups of ions etc) in a mole is 6.022 x 1023 mol-1, this is known as Avogadro’s constant.

For example if you had one mole of sodium metal it would weigh 23g, and in those 23 grams you would find 6.022 x 1023 atoms.

The point is that one mole of any substance, whether it comprises, atoms, molecules or ions, contains this number of particles (whether they be atoms, molecules or ions).