atoms, molecules and stoichiometry

• The molecular formula is the formula that shows the number and type of each atom in a molecule, such as the molecular formula of ethanoic acid which is C H O.
• The molecular formula is found by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula.
• Multiply the number of each element present in the empirical formula by this number to find the molecular formula.
• The empirical formula is calculated from knowledge of the ratio of masses of each element in the compound.
• Simple inorganic molecules have often similar empirical and molecular formulae.
• Ionic compounds always have similar empirical and molecular formulae.
• The empirical formula can be found by determining the mass of each element present in a sample of the compound.
• The empirical formula can also be deduced from data that give the percentage compositions by mass of the elements in a compound.
• The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound, such as the empirical formula of ethanoic acid which is CH O.
• Organic molecules often have different empirical and molecular formulae.
• The rest of the calculation works the same way as the empirical formula calculation.
• A compound that contains water of crystallisation is called a hydrated compound.
• Instead of dividing by atomic masses, you divide by molecular / formula masses.
• A water of crystallisation calculation can be completed in a similar fashion to an empirical formula calculation.
• Cobalt(II) chloride can be hydrated by six or two water molecules CoCl •6H O or CoCl •2H O.
• The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt.
• Instead of elements, you start with the salt and water.
• The degree of hydration can be calculated from experimental results.
• Anhydrous copper(II) sulfate is CuSO.
• The mass of the hydrated salt must be measured before heating.
• The salt is then heated until it reaches a constant mass.
• The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation.
• The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds.
• Water of crystallisation is when some compounds can form crystals which have water as part of their structure.
• Use appropriate state symbols in the fully balanced equation.
• Count the numbers of atoms in each reactant and product to balance the atoms one at a time until all the atoms are balanced.
• In a chemical reaction, the physical state of reactants and products is specified by using state symbols (s) solid (l) liquid (g) gas (aq) aqueous.
• To balance the equation, write out the symbol equation showing reactants and products.
• Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions.
• The ions that do not take part in the reaction are called spectator ions.
• In aqueous solutions, ionic compounds dissociate into their ions.
• An ionic equation shows only the ions or other particles taking part in a reaction, without showing the spectator ions.
• In a chemical reaction, the full chemical equation is Zn (s) + CuSO (aq) → ZnSO (aq) + Cu (s).
• To break down reactants into their respective ions, write the equation as Zn (s) + Cu (aq) → Zn (aq) + Cu (s).
• To cancel the spectator ions on both sides and get the ionic equation, write the equation as 4 4 4 4 4 2+ 4 2 − 2+ 4 2 −.
• When balancing equations, it's important not to change any of the formulae, put the numbers used to balance the equation in front of the formulae, balance the atoms one at a time until all the atoms are balanced, and use appropriate state symbols in the equation.
• Aluminum nitrate has Aluminium in group 13 so has a charge of 3+, while Nitrate is a compound ion and has a charge of 1-.
• There are also more complex negative ions, which are negative ions made up of more than one type of atom.
• The charges of simple ions can be found in a formulae of ionic compounds table.
• The complex ion needs to be placed in a bracket if more than 1 is needed.