Definition: Mole is a unit defined as the amount of substance. It is in the International System of Units (SI) with the symbol "mol". More specifically, a mole is the mass of that substance containing the same number of fundamental units. The number of fundamental units is defined by the Avogadro's number 6.02214076x1023, thus 6.02214076x1023 of atoms, molecules or particle are 1 mol of atoms, molecules or particles.
General formula: The mole formula is related to the Avogadro's constant, which has a value of 6.02214076x1023 mol−1. The mole is used for defining the atomic mass, for example 6.02214076x1023 atoms of carbon has a weight of 12 g, consequently carbon atomic mass is 12 g per mol, as it can be seen in the periodic table. Also, molecular weight is the weight of 6.02214076x1023 molecules and can be calculated using the atomic mass of each atom.
Use: The mole unit is highly used in chemistry. It is the base of the stoichiometry and it is the best option for expressing amounts of reactants and products consumed and formed during a chemical reaction. All the chemical reactions are written as a mole relation. For example in the reaction 2 H2 + O2 → 2H2O, 2 mol of diatomic hydrogen react with 1 mol of diatomic oxygen to form 2 mol of water.
It is also useful for representing concentration units such as the molarity that is a unit of mol L-1(mole per litre) or molecular weight.
Examples: Calculate the mass of 2.45 mol of bismuth knowing that atomic mass of bismuth is 208.98 g/mol.
The atomic mass means 1 mol of bismuth has a mass of 208.98 g. Then:
1 mol → 208.98 g
2.45 mol → X X = (2.45 mol*208.98g)/1 mol = 512.00 g
Calculate how many molecules there are in 4.98 g of gas N2?
The nitrogen atomic mass is 14 g/mol, then the N2 gas, have a mass of 2*14g/mol = 28 g/mol that means in 28 g of gas, there are 6.02214076x1023molecules. Then,
28 g → 6.02214076x1023 molecules
4.98 g → X X = (4.98g*6.02214076x1023 molecules) / 28 g = 1.07x1023molecules
Considerations: It is important to remember that Avogadro's number has a practical purpose, since the weight of 1 atoms or 1 molecule is too small, it is more practical to use the mole unit to express the atomic and molecular weight.