1.2.5 Empirical & Molecular Formulae

Empirical & Molecular Formulae

• The molecular formula is the formula that shows the number and type of each atom in a molecule
• Eg. the molecular formula of ethanoic acid is C2H6O2
• The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
• Eg. the empirical formula of ethanoic acid is CH3O
• Organic molecules often have different empirical and molecular formulae
• Simple inorganic molecules however have often similar empirical and molecular formulae
• Ionic compounds always have similar empirical and molecular formulae

Empirical & Molecular Formulae Calculations

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 give the percentage compositions by mass of the elements in a compound

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 formula 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

Worked example: Calculating molecular formula

• Step 1: Calculate relative formula mass of empirical formula

Relative formula mass = (C x 4) + (H x 10) + (S x 1)

Relative formula mass = (12 x 4) + (1 x 10) + (32 x 1)

Relative formula mass = 90

• Step 2: Divide relative formula mass of X by relative formula mass of empirical formula

Ratio between Mr of X and the Mr of the empirical formula = 180/90

Ratio between Mr of X and the Mr of the empirical formula = 2

• Step 3: Multiply each number of elements by 2

(C4 x 2) + (H10 x 2) + (S1 x 2)     =    (C8) + (H20) + (S2)

Molecular Formula of X is C8H20S2

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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