3.2 Determination of Formulae

3.2 Determination of Formulae

1. Molecular Formulae

   • The number of atoms in each type of element in a molecule
   • Ratio

2. Empirical Formulae

   • The simplest whole-number ratio of atoms of each element in a compound
   • The ratio of atoms or ions in a compound and will always be the same ]

3. Relative Molecular Mass

   • The weighted mean mass of a molecule compared with 1/12th the mass of an atom of carbon-12
   • Can be calculated by adding together the relative atom masses of the elements making up a molecule
   • E.g/ H2O = (2 x 1.0) +16.0 = 18.0
   • E.g/ CH4 = 12.0 + (4 x 1.0) = 16.0
   • E.g/ C6H12O6 = (6 x 12.0) + (12 x 1.0) + (6 x 16.0) = 180.0

4. Relative formula mass

   • The mean mass of a formula unit compared to 1/12th the mass of an atom of carbon-12
   • Calculated by adding together the relaltive atomic masses of the elements in the empirical formula
   • E.g/ NaCl = 23.0 + 35.5 = 58.5
   • E.g/ Ca(NO3)2 = 40.1 + (14.0 + 16.0 x 3) x2 = 164.1

5. Hydrated Salts

   • many coloured crystals are hydrated - water molecules are part of their crystalline structure
     • This is known as water of crystallisation
   • 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
     • This leaves behind white, ANHYDROUS copper(II) sulphate
       • CuSO4 . 5H2O (s) --> CuSO4 (s) + 5H2O (l)
         • 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
   • Formula of Hydrated Salt
     • 1) Weigh an empty crucible
     • 2) Add the hydrated salt into the weighed crucible. Weigh the crucible and the hydrated salt.
     • 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) Leave the crucible to cool. Then weigh the crucible and anhydrous salt
   • Calculating the formula of hydrated salts
     • 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
       • Step 1: Caluclate the amount, in mol, of anhydrous CuSO4.
         • 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
       • Step 2: Calculate the mass and amount, in mol, of water.
         • 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
       • Step 3: Find the smallest whole-number ratio
         • CuSO4
           • 0.0400/0.0400 = 1
         • 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