Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Relative molecular mass
What is Relative molecular mass?
Relative molecular mass, Mr, is the sum of the relative atomic masses of all the atoms shown in the formula of a molecule; for an ionic compound the same sum is called the relative formula mass, and neither quantity has a unit.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Relative molecular mass 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 Relative molecular mass
- "Magnesium oxide has a relative molecular mass of 40." Fix The number is right, the name is not. \(\mathrm{MgO}\) is ionic and has no molecules, so \(40\) is its relative formula mass.
Questions students ask about Relative molecular mass
What is the difference between relative molecular mass and molar mass?
They share a number and nothing else. Relative molecular mass, \(M_r\), is the sum of the relative atomic masses in a formula; it is a comparison with one-twelfth of the mass of a carbon-12 atom, so it is a pure number with no unit. Molar mass, \(M\), is the mass of one mole and carries the unit \(\mathrm{g\,mol^{-1}}\). \(M_r(\mathrm{CO_2}) = 44\) but \(M(\mathrm{CO_2}) = 44\ \mathrm{g\,mol^{-1}}\), and only the molar mass can go into \(n = m/M\). Writing \(M_r = 44\ \mathrm{g}\) loses the mark.