1. Stoichiometry
Chapter 11

2. To calculate the percentage composition by mass.
Example: Calculate the percentage by mass of each element in magnesium oxide.
(a) Relative molecular mass MgO = = 40 24
% Mg = –––– ´ 100 = % 40 16
% O = –––– ´ 100 = %

3. To calculate the empirical formula of a compound from its percentage composition.
The empirical formula of a compound is the formula showing the simplest whole number ratio of the different atoms present in the molecule.

5. To calculate the empirical formula of a compound given the masses of reactants and products.
Example: When 2.7g of aluminium combined with oxygen it was found that 5.1g of Aluminium oxide was formed. What is the Empirical formula of aluminium oxide?
Answer
aluminium + oxygen ¾¾¾® aluminium oxide
2.7 g g
mass of aluminium = 2.7 g mass of aluminium oxide = 5.1 g
mass of oxygen in aluminium oxide = = 2.4 g
mass
number of moles of aluminium in aluminium oxide = –––––––––––––– _____ = 0.1
Rel. atomic mass
mass
number of moles of oxygen in aluminium oxide = ––––––––––––––– ___ = 0.15
Rel. atomic mass
Ratio of Al : O = 0.1 : 0.15 = 2 : 3
Empirical Formula = Al2O3