The Mole

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.