determination of formulae

• The empirical formula is the simplest whole-number ratio of atoms of each element in a compound
• Relative molecular mass $M_r$ compares the mass of a molecule with the mass of an atom of carbon-12 .

  Calculate relative molecular mass by adding together the relative atomic masses of the elements making up the molecule
• Relative formula mass compares the mass of a formula unit with the mass of an atom of carbon-12.

  It is calculated by adding together the relative atomic masses of the elements in the empirical formula
• Many coloured crystals are hydrated - water molecules are part of their crystalline structure. This water is known as water of crystallisation
• When blue crystals of hydrated copper(II) sulfate are heated, bonds holding the water within the crystal are broken and the water is driven off, leaving behind white anhydrous copper(II) sulfate. Without water, the crystalline structure is lost and a white powder remains.

  It is difficult to remove the last traces of water so the anhydrous copper(II) sulfate can appear very pale blue
• Assumptions made when calculating the experimental formula for a hydrated crystal:

  1. All of the water has been lost - if the hydrated and anhydrous forms have different colours, you can be fairly sure when all the water has been removed but you only see the surface of the crystals so some water could be left inside. A good solution is to heat to constant mass - crystals are heated repeatedly until mass of residue no longer changes
  2. No further decomposition - many salts decompose further when heated. E.g. if heated very strongly, copper(II) sulfate decomposes to form black copper (II) oxide