Zusammenfassung der Ressource
3.2 Determination of Formulae
- Molecular Formulae
- The number of atoms in each type of element in a molecule
- Ratio
- 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 ]
- 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
- 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
- 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)
Anmerkungen:
- 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