EL 6 

-     A mole of a substance in 6.023 times 10^23

-       Moles=mass/Mr

-       In a chemical reaction involving 2 reactants it is common to use an excess of one of the reactants to ensure that all the other reactant is used

==-       Limiting reactant=== the reactant that is completely used up as it limits the amount of products

==-       Empirical formular=== the simplest whole number ration of the elements in a compound

==-       Molecular formular=== the actual numbers of different types of atom in a compound

==-       Percentage yield= (experimental yield/theoretical yield) times 100==

==-       Reasons for the percentage yield being less than 100%==

Ø  The reaction might be in an equilibrium system

Ø  Impurities of the reactants

Ø  Loss of products

Ø  Side reactions producing unwanted by-products

-       ==Law of conservation of mass=== the total mass of the products formed in a reaction is equal to the total mass of the reactants. Atoms cannot be created or destroyed in a chemical reaction

-       ==To balance an equation==

Ø  Write down the symbol equation

Ø  Count the number of atoms on each side

Ø  Add numbers Infront of reactants or products to balance each element

==-       Water of crystallisation=== where there are water molecules closely related with substances eg. CuSO4.5H2O

==-       Anhydrous=== there are no associated water molecules