Quantitative chemistry - (CCEA)Empirical formula and molecular formula

We looked at calculating and using moles for solids using mass in unit 1.7. Here we are using moles of a solute dissolved in a solution and moles of a gas to carry out calculations.

Part ofCombined ScienceFurther chemical reactions, rates and equilibrium, calculations and organic chemistry

Empirical formula and molecular formula

The of a compound is the simplest, whole number ratio of atoms of each element in a compound.

The shows the actual number of atoms of each element present in a compound.

Example:

Molecular formulaEmpirical formula
C6H12CH2
C6H6CH
CH4CH4
Molecular formulaC6H12
Empirical formulaCH2
Molecular formulaC6H6
Empirical formulaCH
Molecular formulaCH4
Empirical formulaCH4

Converting empirical formulae to molecular formulae

You can work out the molecular formula from the empirical formula, if you know the relative mass formula (Mr) of the compound.

Add up the atomic masses of the atoms in the empirical formula.

Example:

The empirical formula of a hydrocarbon is CH2 and its Mr is 42.

  • the mass of the atoms in the empirical formula is 14
  • 42 ÷ 14 = 3
  • multiply the numbers in the empirical formula by 3

The molecular formula of the hydrocarbon is C3H6.

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Water of crystallisation

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