Stoichiometry Revision Notes IGCSE Chemistry CIE

Symbols & Formulae

Stoichiometry deals with the application of the laws of definite proportions and of the conservation of mass and energy to chemical activity

For eg; when oxygen and hydrogen react to produce water, one mole of oxygen reacts with two moles of hydrogen to produce two moles of water.

In addition, stoichiometry can be used to find quantities such as the amount of products that can be produced with a given amount of reactants and percent yield.

Symbols & Formulae of Elements & Compounds

Element symbols

Each element is represented by its own unique symbol as seen on the Periodic Table e.g. H is hydrogen
Where a symbol contains two letters, the first one is always in capital letters and the other is small, eg. sodium is Na, not NA
Atoms combine together in fixed ratios that will give them full outer shells of electrons
The chemical formula tells you the ratio of atoms
Eg. H₂O is a compound containing 2 hydrogen atoms which combine with 1 oxygen atom
The chemical formula can be deduced from the relative number of atoms present
Eg. if a molecule contains 3 atoms of hydrogen and 1 atom of nitrogen then the formula would be NH₃
Diagrams or models can also be used to represent the chemical formula

Chemical formulae

The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)
The empirical formula tells you the simplest whole-number ratio of atoms in a compound
The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H₂ has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom

Example: Butane

• Structural formula (simplified)

CH₃CH₂CH₂CH₃

• Molecular formula

C₄H₁₀

• Empirical formula

C₂H₅

 Deducing formulae by combining power

1. The concept of valency is used to deduce the formulae of compounds
2. Valency or combing power tells you how many bonds an atom can make with another atom
3. Carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds

We can use the combining power of each atom to work out a formula

Example: what is the formula of aluminium sulfide?

Write out the symbols of each element and write their combining powers underneath:

Al         S

3          2

The formula is then calculated by cross multiplying each atom with the number opposite, hence the formula for aluminium sulfide is Al₂S₃

Deducing Formulae of Ionic Compounds

• The formulae of these compounds can be calculated if you know the charge on the ions.
• Below are some common ions and their charges:

For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0

Example: what is the formula of aluminium sulfate?

Write out the formulae of each ion, including their charges

Al³⁺ SO₄^2-

Balance the charges by multiplying them out:Al³⁺ x 2 = +6 and SO₄^2- x 3 = -6; so +6 – 6 = 0

So the formula is Al₂(SO₄)₃

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