IB Chemistry - Stoichiometry

IB Chemistry home > Syllabus 2016 > Stoichiometry > Mass and moles

Syllabus ref: 1.2

The nature, or properties, of a pure substance is determined by the type of particles that it contains. These may be atoms, molecules or ions.

Ionic solids contain a giant lattice (network) of ions, simple covalent solids contain discrete molecules held together by loose forces and giant covalent structures have a lattice (network) of atoms all joined together by strong covalent bonds. Metals may be considered to be a regular arrangement of atoms for the purpose of mass calculations.


The terminology used depends on the type of structure under consideration. There are three ways to describe the relative mass of solids:

Where each of these applies is considered below.

1. Relative atomic mass

Relative atomic mass is used to define the relative mass of one atom of a metallic element, or non-metallic element, which may be considered to consist of unique atoms for the purposes of calculations.

Inert gases
Relative atomic mass
Neon - Ne 20
Argon - Ar 40
Giant covalent elements
Relative atomic mass
Boron - B 10.8
Silicon - Si 28
Carbon - C 12
Other non-metallic elements
Relative atomic mass
Phosphorus - P 31
sulfur - S 32
Relative atomic mass
Sodium - Na 23
Magnesium - Mg 24
Iron - Fe 56
Uranium - U 238

Note: Although sulfur and phosphorus contain discrete molecules (S8 and P4, respectively), they are usually treated in calculations as monatomic.

2. Relative molecular mass

Relative molecular mass is used to define the relative mass of one molecule of a covalent substance, whether element or compound.

Molecular elements
Relative molecular mass
Hydrogen - H2 2
Nitrogen - N2 28
Oxygen - O2 32
Fluorine - F2 38
Chlorine - Cl2 71
Bromine - Br2 160
Iodine - I2 254
simple covalent compounds Relative molecular mass
Ammonia - NH3 17
Methane - CH4 16
Water - H2O 18
Hydrogen chloride - HCl 36.5
Glucose - C6H12O6 180
Carbon dioxide- CO2 44

Note The halogens bromine and iodine are liquid and solid respectively, at room temperature and pressure.

3. Relative formula mass

Relative formula mass is used to define the mass of the simplest ratio of particles in ionic and giant covalent compounds.

Ionic compounds
Relative formula mass
sodium chloride - NaCl 58.5
magnesium oxide - MgO 40
potassium nitrate - KNO3 101
calcium carbonate - CaCO3 100
Giant covalent compounds Relative formula mass
Silicon dioxide - SiO2 60


The Mole

The amount of substance that contains an Avogadro number of particles (6.02 x 1023) is called 1 mole of that substance. Where the word amount is used it always refers to the number of moles.

metals and other elements
Moles = mass / relative atomic mass
Simple covalent substances
Moles = mass / relative molecular mass
Ionic and giant covalent compounds
Moles = mass / relative formula mass

All of these relationships can be summarised as an equation triangle

Example 1: Calculate the number of moles in 12 g of magnesium

Magnesium has a relative atomic mass = 24

Moles of magnesium in 12 g = 12/24 = 0.5 moles

Example 2: Calculate the amount represented by 15g of silicon dioxide

Silicon dioxide, SiO2, is a macromolecular structure with a relative formula mass = 60

Moles of silicon dioxide in 15 g = 15/60 = 0.25 moles