Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Formula unit
What is Formula unit?
A formula unit is the simplest whole-number ratio of ions in an ionic compound, written so that the total positive charge exactly cancels the total negative charge; magnesium chloride, MgCl2, is one formula unit containing one magnesium ion and two chloride ions.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Formula unit in context
The mole (mol) is the unit of amount of substance. One mole contains \(6.02\times10^{23}\) particles — the Avogadro constant — and you must always name which particles: atoms, ions, molecules or formula units. Amount of substance is calculated as mass divided by molar mass, \(n = m/M\), where molar mass \(M\) is measured in \(\mathrm{g\,mol^{-1}}\). Relative atomic mass \(A_r\) and relative molecular mass \(M_r\) share the same numbers as molar mass but carry no unit, because each is a comparison with one-twelfth of the mass of a carbon-12 atom. One mole of any gas occupies \(24\,\mathrm{dm^3}\) at room temperature and pressure.
Common mistakes with Formula unit
- "\(\mathrm{MgO}\) is a molecule of magnesium oxide." Fix Magnesium oxide is a giant ionic lattice with no discrete molecules. \(\mathrm{MgO}\) is a formula unit — the simplest ratio of ions — which is why its relative mass is called a relative formula mass.