3.2 Determination of Formulae

Molecular Formulae
1. The number of atoms in each type of element in a molecule
2. Ratio
Empirical Formulae
1. The simplest whole-number ratio of atoms of each element in a compound
2. The ratio of atoms or ions in a compound and will always be the same ]
Relative Molecular Mass
1. The weighted mean mass of a molecule compared with 1/12th the mass of an atom of carbon-12
2. Can be calculated by adding together the relative atom masses of the elements making up a molecule
3. E.g/ H2O = (2 x 1.0) +16.0 = 18.0
4. E.g/ CH4 = 12.0 + (4 x 1.0) = 16.0
5. E.g/ C6H12O6 = (6 x 12.0) + (12 x 1.0) + (6 x 16.0) = 180.0
Relative formula mass
1. The mean mass of a formula unit compared to 1/12th the mass of an atom of carbon-12
2. Calculated by adding together the relaltive atomic masses of the elements in the empirical formula
3. E.g/ NaCl = 23.0 + 35.5 = 58.5
4. E.g/ Ca(NO3)2 = 40.1 + (14.0 + 16.0 x 3) x2 = 164.1
Hydrated Salts
1. many coloured crystals are hydrated - water molecules are part of their crystalline structure
  1. This is known as water of crystallisation
2. When blue crystals of hydrated copper(II) sulphate are heated, bonds holding th water within the crystal are broken and the water is driven off
  1. This leaves behind white, ANHYDROUS copper(II) sulphate
    1. CuSO4 . 5H2O (s) --> CuSO4 (s) + 5H2O (l)
      Annotations:
      - The CuSO4.5H2O is the hydrated copper(II) sulphate, as it is copper sulphate WITH 5 molecules of water, which is represented by the ".5H2O". The anhydrous product is the CuSO4 as that is just Copper(II) Sulphate
3. Formula of Hydrated Salt
  1. 1) Weigh an empty crucible
  2. 2) Add the hydrated salt into the weighed crucible. Weigh the crucible and the hydrated salt.
  3. 3) Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about one minute. Then head it strongly for a further 3 minutes
  4. 4) Leave the crucible to cool. Then weigh the crucible and anhydrous salt
4. Calculating the formula of hydrated salts
  1. Lets say the results you've gathered from you water of crystallisation experiment are: Mass of Crucible/g : 18.742 Mass of Crucible and hydrated salt/g : 28.726 Mass of crucible and Anhydrous salt/g = 25.126
    1. Step 1: Caluclate the amount, in mol, of anhydrous CuSO4.
      1. Mass, m, of CuSO4 formed = (Mass of crucible + anhydrous salt) - (Mass of crucible)
        = 25.126 - 18.742 = 6.384 grams
        Moles = mass/mr = 6.384 / 159.6 = 0.0400 mol
    2. Step 2: Calculate the mass and amount, in mol, of water.
      1. Mass, m, of H2O formed = (Mass of crucible + Hydrated salt) - (Mass of crucible and Anhydrous salt)
        = 28.726 - 25.126 = 3.600grams
        Number of moles of H2O = 3.600/18.0 = 0.200mol
    3. Step 3: Find the smallest whole-number ratio
      1. CuSO4
        0.0400/0.0400 = 1
      2. H2O
        0.200/0.0400 = 5
        This means there is a 1:5 ratio. So the formula of the hydrated Copper(II) Sulphate is CuSO4.5H2O

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3.2 Determination of Formulae

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	Created by Bee Brittain about 8 years ago