Empirical Formulae & Formulae of Ionic Compounds | CIE IGCSE Chemistry Revision Notes 2023

Empirical formulae

Extended Tier Only

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
The empirical formula of an organic molecule is often different to its molecular / chemical formula
- For example, ethanoic acid has the chemical formula CH₃COOH or C₂H₄O₂ but its empirical formula is CH₂O
The molecular / chemical formula of an ionic compound is always its empirical formula
- For example, sodium chloride has the chemical formula NaCl, which is also its empirical formula

Worked example

Complete the table to give the molecular and empirical formulae of the given compounds.

Chemical	Molecular formula	Empirical formula

Answers:

The completed table is:

Chemical	Molecular formula	Empirical formula
	C₃H₈	C₃H₈
	C₂H₄	CH₂
	C₄H₁₀O	C₄H₁₀O

The first compound contains 3 carbon atoms and 8 hydrogen atoms
- This 3:8 ratio of atoms cannot be simplified
- Therefore, the molecular and empirical formula are both C₃H₈
The second compound contains 2 carbon atoms and 4 hydrogen atoms
- This 2:4 ratio of atoms can be simplified to 1:2
- Therefore, the molecular formula is C₂H₄ and the empirical formula is CH₂
The third compound contains 4 carbon atoms, 10 hydrogen atoms and 1 oxygen atom
- This 10:4:1 ratio of atoms cannot be simplified
- Therefore, the molecular and empirical formula are both C₄H₁₀O

Deducing formulae of ionic compounds

Extended Tier Only

Metals and non-metals react together to form ionic compounds
- Ionic compounds are not simple molecules
- Remember: Simple molecules are formed when non-metals react together to form compounds
Ionic compounds involve the metal losing electrons and the non-metal gaining electrons to form ions

Some ions that you will be expected to be able to use, because they are stated in the exam specification, include:
- Hydrogen ions, H⁺ - sometimes referred to as protons
- Group 1 ions, e.g. Li⁺, Na⁺, K⁺
- Group 7 ions, F^–, Cl^–, Br^–
- Copper(II) ions, Cu²⁺
- Iron(II) ions, Fe²⁺
- Iron(III) ions, Fe³⁺
There are some polyatomic (containing more than one atom) ions stated in the exam specification:
- Carbonate ions, CO₃^2–
- Sulfate ions, SO₄^2–
- Hydroxide ions, OH^–
- Nitrate ions, NO₃^–
- Ammonium ions, NH₄⁺

Determining the formulae of ionic compounds

Ionic compounds typically have no overall charge
- This means that the size of any positively charged ion is cancelled by the size of any negatively charged ion
- Careful: This should not be confused with an atom having no overall charge

Direct comparison

The formula of an ionic compound can be determined by directly comparing the charges of the ions:
- For example, iron(II) sulfate
  - The iron(II) ion is Fe²⁺, which means that it has a 2+ or +2 charge
  - The sulfate ion is SO₄^2–, which means that it has a 2– or –2 charge
  - The charges cancel each other out
    - Mathematically, (+2) + (–2) = 0
  - This means that one SO₄^2– ion is needed to cancel the +2 charge on Fe²⁺
  - Therefore, the formula of iron(II) sulfate is FeSO₄

The swap-and-drop method

When the ions in the ionic compound have different charges, it can be easier to use the swap-and-drop method
- Careful: If you use this method with ions that have the same charge, then you must give the simplest whole number ratio to get the correct answer
For example, copper(II) chloride:
- The copper(II) ion is Cu²⁺, which means that it has a 2+ or +2 charge
- The chloride ion is Cl^–, which means that it has a 1– or –1 charge
- The size of the charge on the copper(II) ion indicates the number of chloride ions needed, and the size of the charge on the chloride ion indicates the number of copper(II) ions needed

Determining the formula of copper(II) chloride

~5RmSBVa_copperii-chloride-swap-and-drop

The charges swap from element to element and drop down. The positive and negative signs are removed and there is no need for the number 1.

- - This gives the overall formula of copper(II) chloride as CuCl₂

Worked example

The compound produced in the reaction between iron wool and chlorine contains the ions Fe³⁺ and Cl^–.

Give the formula of this compound.

State the name of this compound.

Answers:

Part a)

Direct comparison method:
- The iron ion is Fe³⁺, which means that it has a 3+ or +3 charge
- The chloride ion is Cl^–, which means that it has a 1– or –1 charge
- The charges do not cancel each other out
  - Mathematically, (+3) + (–1) ≠ 0
- Three Cl^– ions are needed to cancel the +3 charge on Fe³⁺
- Therefore, the formula is FeCl₃
Swap-and-drop method

The formula is FeCl₃

Part b)

The metal is iron and the chlorine will change to chloride
Therefore the name is iron chloride

Exam Tip

Take your time determining the chemical formula of ionic compounds with

Different charges on the ions
Polyatomic ions

Empirical Formulae & Formulae of Ionic Compounds (CIE IGCSE Chemistry)

Revision Note