Chemical formulae, equations and calculations

• To calculate Mr from Ar, add the Ar of the relevant elements together
• Mol is used for the amount of substance used
• To find Ar, multiply the mass by the abundance of each isotope, add them together, and divide by 100
• To calculate percentage yield, divide the actual mass by the theoretical mass and multiply by 100

  (actual/theoretical)*100
• To calculate reacting masses:

  • find the moles of the mass of the substance
  • use the molar ratio to deduce the amount in moles of whatever it is
  • calculate the mass of the moles
• To find the formula of compounds experimentally:

  • weigh a crucible and lid
  • put the substance in the crucible, replace the lid and reweigh
  • calculate the mass of the substance
  • Heat the crucible until the reaction is complete
  • allow to cool and reweigh
  • repeat the heating and cooling until two consecutive masses are the same
  • calculate the mass of product formed
• Empirical formula is the simplest whole number ratio between atoms/ions in a compound
• Molecular formula shows the actual number of atoms of each type of element in a molecule
• To calculate empirical formula from experimental data:

  • put the symbols for each element at the top of the page and write the mass underneath
  • divide the mass by the Ar
  • divide all the numbers by the smallest value to give the whole number ratio
• To calculate molecular formula from experimental data:

  • calculate empirical formula relative mass
  • Find out the number of times the relative mass of the empirical formula goes into the Mr
  • this tells us how much bigger the molecular formula is than the empirical formula
• Metal oxides can be obtained experimentally through reacting a metal with oxygen (burning) and recording mass changes
• To obtain salts of the water if crystallisation heat or leave hydrous copper sulphate to evaporate the water, creating anhydrous copper sulphate