The mole is the unit for amount of substance. The number of particles in a substance can be found using the Avogadro constant. The mass of product depends upon the mass of limiting reactant.

The actual masses of atoms and molecules are too small to be of practical use in calculations. Therefore a special quantity, called a mole, is used.

In terms of mass, a mole (mol) is the relative atomic mass (*A*_{r}) of an element in grams, or the relative formula mass (*M*_{r}) in grams if it's a compound. Examples for elements and compounds are given in the table below, along with its formula, relative formula mass (RFM) and the mass of 1 mole.

Example | Formula | RFM | Mass of 1 mole |
---|---|---|---|

Calcium | Ca | 40 | 40 g |

Oxygen | O_{2} | 2 x 16 = 32 | 32 g |

Calcium carbonate | CaCO_{3} | 40 + 12 + (3 × 16) = 100 | 100 g |

As the mole quantity is based on the relative masses of atoms, there is a relationship between a mole of a substance and the number of particles it contains. As atoms and molecules are so small the number in a mole is very large. This number is called the Avogadro constant, and it is equal to: 602,000,000,000,000,000,000,000 (= 6.02 × 10^{23}) formulae units.

The Avogadro constant is defined as: the number of carbon atoms in exactly 12 g of carbon-12, ^{12}C.

One mole, 1 mol, of a substance is defined as: the Avogadro constant number of particles (which is 6.02 × 10^{23} atoms, molecules, formulae or ions of that substance).

For example:

- the formula of methane is CH
_{4} - the relative atomic mass (
*A*_{r}) of C = 12 - the
*A*_{r}of H = 1 - so the relative formula mass (
*M*_{r}) of CH_{4}= 12 + (4 × 1) = 16 - so 1 mole of methane = 16 g of methane and contains 6.02 × 10
^{23}molecules - 16 g of methane = 6.02 × 10
^{23}as there are 5 atoms in each formulae (molecule) - 16 g of methane = 5 × 6.02 × 10
^{23}= 3.01 × 10^{24}atoms

The number of particles of a substance can be calculated using:

- the Avogadro constant number (6.02 × 10
^{23}) - the amount of substance in mole

Number of particles = 6.02 × 10^{23} × mol

Calculate the number of water molecules in 0.5 mol of water.

Number of water molecules = 6.02 10^{23} × 0.5

= 3.01 × 10^{23}

It is important to be clear about the particles involved. For example, 3.01 × 10^{23} water molecules contain 9.03 × 10^{23} atoms. This is because a water molecule, H_{2}O, contains three atoms.

- Question
Calculate the number of oxygen atoms in 0.5 mol of oxygen molecules, O

_{2}.Number of atoms = 2 × 0.5 × 6.02 × 10

^{23}= 6.02 × 10

^{23}