Empirical & Molecular Formula (CIE IGCSE Chemistry)

EXTENDED

Calculating Empirical Formula

• The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
  • E.g. the empirical formula of ethanoic acid is CH₂O
• Organic molecules often have different empirical and molecular formulae
• The formula of an ionic compound is always an empirical formula 

Answer:

	hydrogen	oxygen
Write the mass of each element	10 g	80 g
Divide each mass by the relative atomic mass to find the number of moles	10/1 = 10	80/16 = 5
Find the molar ratio by dividing by the smallest number	10/5 = 2	5/5 = 1

Empirical formula = H₂O

   Answer:

	carbon	hydrogen	oxygen
Convert % to g by assuming 100 g of substance is present	31.58 g	5.26 g	63.16 g
Divide each mass by the relative atomic mass to find the number of moles in 100 g	31.58/12 = 2.63	5.26/1 = 5.26	63.16/16 = 3.95
Find the molar ratio by dividing by the smallest number	2.63/2.63 = 1	5.26/2.63 = 2	3.95/2.63 = 1.5
Multiply all by 2 to obtain a whole number ratio	2	4	3

Empirical formula = C₂H₄O₃

Calculating Molecular Formula

• Molecular formula gives the actual numbers of atoms of each element present in the formula of the compound
• To calculate the molecular formula:
  • Step 1: Find the relative formula mass of the empirical formula
  • Step 2: Use the following equation:

• • Step 3: Multiply the number of each element present in the empirical formula by the number from step 2 to find the molecular formula

Table showing the Relationship between Empirical and Molecular Formula

Answer

Step 1 - Calculate the relative formula mass of the empirical formula

M_r =  (12 x 4) + (1 x 10) + (32 x 1)   =   90

Step 2 - Divide relative formula mass of X by relative formula mass of empirical formula

180 / 90 = 2

Step 3 - Multiply each number of elements by 2

(C_{4 x 2}) + (H_{10 x 2}) + (S_{1 x 2})     

Molecular Formula of X = C₈H₂₀S₂

Deducing formulae of hydrated salts

• The formula of hydrated salts can be determined experimentally by weighing a sample of the hydrated salt, heating it until the water of crystallisation has been driven off, then reweighing the now anhydrous salt
• From the results, you can determine the mass of anhydrous salt and the mass of the water of crystallisation
• Applying a similar approach to deducing empirical formulae, the formula of the hydrated salt can be calculated

	CuSO4	H2O
Deduce the mass of water of crystallisation and anhydrous salt	= mass of salt after heating = 7.19 g	mass of hydrated salt - mass of anhydrous salt: 11.25 - 7.19 = 4.06 g
Divide each mass by the relative formula mass to find the number of moles
Find the molar ratio by dividing by the smallest number

Therefore the formula of hydrated copper(II) sulfate is CuSO₄.5H₂O