Empirical Formula | Chemistry for Grade 11 (IGCSE) PDF Download

Empirical Formulae

• Example: The molecular formula of ethanoic acid is C₂H₄O₂.
• The empirical formula represents the simplest whole number ratio of atoms for each element found in one molecule or formula unit of the compound. 
  • Example: The empirical formula of ethanoic acid is CH₂O. 
• In the case of organic molecules like ethanoic acid, there can be variations between empirical and molecular formulae. 
• Notably, the formula of an ionic compound always aligns with an empirical formula.

Deducing Formulae of Ionic Compounds

The formulas of these compounds can be determined by understanding the charge carried by the ions involved.
The Periodic Table provides guidance on determining the charge of many elements:

• Group I elements typically form ions with a 1+ charge.
• Group II elements typically form ions with a 2+ charge.
• Group III elements typically form ions with a 3+ charge.
• Group V elements typically form ions with a 3- charge.
• Group VI elements typically form ions with a 2- charge.
• Group VII elements typically form ions with a 1- charge.

Additionally, there are other common ions and their associated charges listed below. It's important to note that a Roman numeral next to the element indicates the charge on the ion, such as copper(II) ions having a charge of 2+. Some chemists refer to these as polyatomic ions.
Common Ions & Their Charges Table

• The net charge of an ionic compound must be zero, necessitating the determination of ion ratios.
• When writing compound ion formulas, brackets are crucial for ions needing replication; for instance, copper(II) hydroxide is denoted as Cu(OH)₂.