Formulae, Equations and Calculations

• Mr:

  • relative formula mass
  • all relative atomic masses added together
• Moles = mass / Mr
• mass:

  • write balanced equation
  • work out Mr of reactant and product
  • find out moles
  • used balanced equation for moles of other substance
  • use moles to calculate mass
• percentage yield

  • (actual yield / percentage yield) x 100
• Empirical formula:

  • smallest whole number ratio of atoms in a compound
  • list all elements by compounds
  • underneath right experimental masses
  • divide mass by Ar for the moles
  • turn numbers into simple ratio by divided by the smallest moles
  • put ratio in the simplest form
• molecular formula tells you actual number of atoms of each element in a single molecule

  • whole number multiples of empirical formulae
  • find mass of empirical formula
  • relative molecular mass divided by mass of empirical formula
  • times that by the empirical formula
• Empirical formulae using combustion:

  • combustion = substance reacts with oxygen
  • heat crucible
  • to remove traces of oil or water
  • leave to cool
  • weigh crucible with lid
  • add clean magnesium ribbon to crucible
  • reweigh
  • mass of magnesium = reading - initial reading for the mass of the crucible and lid
  • heat crucible containing magnesium
  • put lid on to stop solid from escaping with small gap to allow oxygen in
  • heat until magnesium turns white
  • allow to cool and reweigh crucible with lid and contents
  • mass of magnesium oxide is this reading minus initial reading for the mass of the crucible and lid
• Empirical formula using reduction:

  • use Bunsen burner to heat Copper (II) Oxide for about 10 mins
  • solid changes form black to brownish-pink
  • leave test tube to cool
  • weigh test tube with bung and contents
• Empirical formula using reduction:

  • place rubber bung with a hole in middle into a test tube with small hole in the end
  • weigh it
  • take out bung and spread small amount of copper (II) oxide in the middle
  • reinsert bung and weigh it again
  • expel air from test tube by gently turning on gas
  • after 5s, light gas by holding a burning splint next to the hole in the end of a test tube
  • control size of flame by controlling amount of gas flowing through
• Water of Crystallisation:

  • first work out what mass of the hydrated and anhydrous substance
  • calculate number of moles of water lost
  • mass of water lost divide by Mr of H2O
  • calculate moles of anhydrous salt made
  • mass of salt divided by Mr of salt
  • work out ratio of moles of anhydrous salt to moles of water and how much salt is needed for the substance
  • x must be a whole number