Determining Formulae (OCR A Level Chemistry)

• The molecular formula shows the number and type of each atom in a molecule
  • E.g. the molecular formula of ethanoic acid is C₂H₄O₂

• The empirical formula shows the simplest whole number ratio of the elements present in one molecule of the compound
  • E.g. the empirical formula of ethanoic acid is CH₂O

Answer

Empirical formula

• Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
• It is calculated from knowledge of the ratio of masses of each element in the compound
• The empirical formula can be found by determining the mass of each element present in a sample of the compound
• It can also be deduced from data that gives the percentage compositions by mass of the elements in a compound

• The above example shows how to calculate empirical formula from the mass of each element present in the compound
• The example below shows how to calculate the empirical formula from percentage composition

Molecular formula

• The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound
• The molecular formula can be found by dividing the relative molecular mass of the molecular formula by the relative formula mass of the empirical formula
• Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Answer

Step 1: Calculate relative mass of the empirical formula

• • Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
  • Relative empirical mass = (12.0 x 4) + (1.0 x 10) + (32.1 x 1)
  • Relative empirical mass = 90.1

Step 2: Divide relative molecular mass of X by relative empirical mass

• • Ratio between M_r of X and the M_r of the empirical formula = 180.2 / 90.1
  • Ratio between M_r of X and the M_r of the empirical formula = 2

Step 3: Multiply each number of elements by 2

• • (C₄ x 2) + (H₁₀ x 2) + (S x 2)
  • (C₈) + (H₂₀) + (S₂)
  • Molecular formula of X is C₈H₂₀S₂

• Water of crystallisation is when some compounds can form crystals which have water as part of their structure
• A compound that contains water of crystallisation is called a hydrated compound
• The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
  • E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O

• A compound which doesn’t contain water of crystallisation is called an anhydrous compound
  • E.g. anhydrous copper(II) sulfate is CuSO₄

• A compound can be hydrated to different degrees
  • E.g. cobalt(II) chloride can be hydrated by six or two water molecules
  • CoCl₂ ∙6H₂O or CoCl₂ ∙2H₂O

• The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt:

• The degree of hydration can be calculated from experimental results:
  • The mass of the hydrated salt must be measured before heating
  • The salt is then heated until it reaches a constant mass
  • The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation

Answer