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? What about more than one radical unit? Now work out Calcium hydroxide… ….. Combining ionic compounds • The formula can be found from the ions present. • When these ions combined, they must cancel each other out. • AKA must be neutral. • Sodium and chlorine. • Copper(II) Chloride