Stoichiometry: 3.31 - Mass and moles

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.

Syllabus reference

Structure 1.4.2 - Masses of atoms are compared on a scale relative to ¹²C and are expressed as relative atomic mass Ar and relative formula mass Mr .

Determine relative formula masses Mr from relative atomic masses Ar .

Guidance

Relative atomic mass and relative formula mass have no units.
The values of relative atomic masses given to two decimal places in the data booklet should be used in calculations.

Tools and links

Structure 3.1 - Atoms increase in mass as their position descends in the periodic table. What properties might be related to this trend?

Terminology
The mole

Terminology

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

Relative atomic mass (abbreviated A^r)
Relative molecular mass (abbreviated M^r)
Relative formula mass (abbreviated M^r)

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
Metals	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

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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

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