# Stoichiometry presentation ```Stoichiometry
What is that?
• About the relative quantities of reactants and products
in a chemical reaction.
• All elements are made up from simple building blocks
of elements.
• Chemical formulae include:
-the symbols for each element in the compound
-the number of atoms of each element in a unit of
the compound
• We need to be able to work out the chemical formulae
for different compounds.
• E.g Magnesium oxide.
Formulae of Simple Compounds
Molecular formula from valency
Group Number
1
2
3
4
5
6
7
8
Typical Valency (Combining power)
1
2
3
4
3
2
1
0
NaCl
MgCl2
AlCl3
CH4
NH3
H2O
HCl
PCl5
SO3
Example
Example: The formula of a compound formed between carbon and sulphur
Write down the symbols of both the elements involved
C
S
4
2
Swap the valencies
2
4
Divide both by smaller of the two numbers
1
2
Below each symbol write, its valency
Write the formula
1. Magnesium and oxygen
2. Aluminium oxide
3. Potassium oxide
4. Carbon disulfide.
CS2
Compounds containing more than two elements
• Known as Radicals – exist
bonded together
• Each have a specific
combining power.
• What is the chemical
formula for copper sulphate
And
Potassium nitrate?
Calcium carbonate?