The Mole

*Please note: you may not see animations, interactions or images that are potentially on this page because you have not allowed Flash to run on S-cool. To do this, click here.*

The Mole

The relative atomic mass, Ar, of an elementis the average mass of the naturally occurring isotopes of the element relative to the mass of an atom of carbon-12.

The relative molecular mass, Mr, is the mass of a molecule relative to the mass of an atom of carbon-12, which is given the exact mass of 12.

Example: NH3 = 14 + 3x1 = 17.

The relative formula mass is used for compounds made up of ions.

A mole of substance is the amount of substance that has the same number of particles as there are in 12.00g of carbon-12. The particles may be atoms, molecules, ions or even electrons.

This number of particles is a constant known as Avagadros constant (L), and is approximately 6 x 1023mol-1.

The mass of one mole of a substance is often referred to as the molar mass (M). The units of molar mass are gmol-1.

To find the amount of substance present in a given mass, we must divide that mass by the molar mass of the substance.

For example - if we had 10g of CaCO3:

M is 40 + 12 + 3x16 = 100g.

So in 10g of calcium carbonate there is 10/100 mol = 0.1 mol CaCO3.

To find the mass of a given substance, we multiply the number of moles of the substance by the molar mass.

If we know the mass of a reactant, we can find the mass of a product in a chemical equation.

Consider the formation of ammonia from hydrogen and nitrogen.

2N2 (56g) + 3H2 (6g) → 4NH3 (34g)

Hence, for every 6g of hydrogen we make 34g of ammonia.

Just click "Find out more" and get £10 off your first tutorial