In terms of mass, a mole (mol) is the relative atomic mass (Ar) of an element in grams, or the relative formula mass (Mr) 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|
|Oxygen||O2||2 x 16 = 32||32 g|
|Calcium carbonate||CaCO3||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 × 1023) formulae units.
The Avogadro constant is defined as: the number of carbon atoms in exactly 12 g of carbon-12, 12C.
The number of particles of a substance can be calculated using:
Number of particles = 6.02 × 1023 × mol
Calculate the number of water molecules in 0.5 mol of water.
Number of water molecules = 6.02 1023 × 0.5
= 3.01 × 1023
It is important to be clear about the particles involved. For example, 3.01 × 1023 water molecules contain 9.03 × 1023 atoms. This is because a water molecule, H2O, contains three atoms.
Calculate the number of oxygen atoms in 0.5 mol of oxygen molecules, O2.
Number of atoms = 2 × 0.5 × 6.02 × 1023
= 6.02 × 1023