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

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The molecular formula is the formula that shows the number and type of each atom in a molecule

• E.g. the molecular formula of ethanoic acid is C₂H₄O₂

• 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, such as ethanoic acid, often have different empirical and molecular formulae
• The formula of an ionic compound is always an empirical formula

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• The formulae of these compounds can be calculated if you know the charge on the ions
• The Periodic Table can help work out the charge on many elements:
  • Group I elements form ions with a 1+ charge
  • Group II elements form ions with a 2+ charge
  • Group III elements form ions with a 3+ charge
  • Group V elements form ions with a 3- charge
  • Group VI elements form ions with a 2- charge
  • Group VII elements form ions with a 1- charge
• Below are some other common ions and their charges
• Note that a Roman numeral next to the element tells you the charge on the ion, e.g. copper(II) ions have a charge 2+
• There are several common compound ions included in the table 
  • Some chemists call these polyatomic ions

Common Ions & Their Charges Table

 

• The overall sum of the charges of an ionic compound should be 0 
• You therefore need to work out the ratio of the ions to ensure this is the case 
• When you write the formula of a compound ion it is necessary to use brackets around the compound ion where more than one of that ion is needed in the formula
  • For example copper(II) hydroxide is Cu(OH)₂

Answer 1

Symbol	Na                 Br
Ion charge	1+                 1-
Balance the number of ions	1 sodium ion is needed for each bromide ion
Ratio of ions	1:1
Formula	NaBr

Answer 2

Symbol	Al                 F
Ion charge	3+                 1-
Balance the number of ions	3 fluoride ions are needed for each aluminium ion
Ratio of ions	1:3
Formula	AlF3

Answer 3

Symbol	Al                 O
Ion charge	3+                 2-
Balance the number of ions	2 aluminium ions are needed for 3 oxide ions
Ratio of ions	2:3
Formula	Al2O3

Answer 4

Symbol	Mg2+                  NO3-
Ion charge	2+                       1-
Balance the number of ions	2 nitrate ions are needed for each magnesium ion
Ratio of ions	1:2
Formula	Mg(NO3)2

Answer 5

Symbol	NH4+                 SO42-
Ion charge	1+                     2-
Balance the number of ions	2 ammonium ions are needed for each sulfate ion
Ratio of ions	2:1
Formula	(NH4)2SO4