Stoichiometry is the calculation of reactants and products in chemical reactions. Masses of the reactants/products can be measured in grams or moles. A mole of a substance is the amount that contains the same number of units as the number of carbon atoms in 12 grams of carbon-12.

The Avogadro constant

12 g of carbon-12 contains 602 000 000 000 000 000 000 000 carbon atoms! This number is called the Avogadro constant and it is usually written in a short way as 6.02 x 1023. The mole can also be described as the amount of substance that contain 6.02 x 1023 particles. The particles can either be atoms, ions, or molecules depending on the nature of the substance.

Molar mass, Relative atomic mass and Relative mloecular mass

The relative atomic mass(Ar) of an element is the mass of the element is compared with the mass of 1 atom of the carbon-12 atom. The Ar of a substance is roughly equal to the nucleon number (the total number of neutrons and protons in an atom).

Ar of some common elements

The relative molecular mass of a compound (Mr) is the mass of the compound compared to the mass of the mass of the carbon-12 atom. It is called the relative formula mass if the compound is ionic. It is equal to the sum of the relative atomic masses of the atoms that make up the compound. However elements such as Hydrogen, Nitrogen, Chlorine naturally exist as molecules of 2 atoms each which means
they have Mr which is twice the Ar. They are said to be diatomic.

Mr of some common diatomic elements
Mr of some common compounds
SubstanceFormulaMr of the compund
Sodium ChlorideNaCl23 + 35.5 = 58.5
Sulphuric AcidH2SO42(1) + 32 + 4(16) = 98
MethaneCH412 + 4(1) = 16

The mass of 1 mole of a compound is called its molar mass and it is equal to the relative atomic mass(Ar) or relative molecular mass (Mr) of a substance. It is numerically equal to the Mr or Ar expressed in grams. The relative molecular mass of water is 18 and so its molar mass is 18 g.

Calculations involving moles

Mass of a given substance = Number of moles x mass of 1 mole

Empirical formula

The empirical formula is a formula which shows the simplest ratio in which atoms are combined. Different compounds with the same atomic ratio e.g C2H4, C3H6, C4H8 and C5H10 have the same empirical formula ( CH2 in this case). All of these formulae show the same ratio of carbon atoms to hydrogen atoms.

Example 1

64 grams of sulfur combine with 64 grams of oxygen to form an oxide of sulfur. What is its empirical formula?


Tabulate your calculation as follows
Relative atomic masses (Ar)3216
Number of moles64/32 = 264/16 = 4
Molar ratio (devide by the smallest number of moles)2/2 = 14/2 = 2

Therefore the ratio of Sulphur to Oxygen is 1:2 and so the empirical formula of the oxide that forms is SO2.

Example 2

It is given that compound A contains 80% carbon and 20% hydrogen. What is its empirical formula?


Y contains 80% carbon and 20% hydrogen. So 100 g of A contains 80 g of carbon and 20 g of hydrogen

Tabulate your calculation as follows
Relative atomic masses (Ar)121
Number of moles80/12 = 6.6720/1 = 20
Molar ratio (devide by the smallest number of moles)6.67/6.67 = 120/6.67 = 3

Therefore the empirical formula of A is CH3.

Molecula formula

The molecular formula shows the actual numbers of atoms that
combine to form a molecule. For some compounds the empirical formula is the same as molecular formula.

Formulae of some compounds
CompoundMolecular formulaEmpirical formula


The concentration of a solution is the amount of solute, in grams or moles, that is dissolved in a unit volume of solution. When 1 mole of sodium chloride is dissolved in 1 dm3 (1000cm3) of water the concentration of the solution is 1 mol/dm3.