Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Molar mass
What is Molar mass?
Molar mass, M, is the mass of one mole of a substance, measured in grams per mole; its numerical value matches the relative molecular or formula mass, but unlike those quantities it carries a unit and is the value used in the relationship amount of substance equals mass divided by molar mass.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Molar 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 Molar mass
- 3. Giving \(A_r\) or \(M_r\) a unit Repair Both are comparisons against one-twelfth of a carbon-12 atom, so both are pure numbers. Writing \(M_r = 44\ \mathrm{g}\) is wrong; the quantity that carries \(\mathrm{g\,mol^{-1}}\) is molar mass. Lesson 3.2
- 4. Treating \(M_r\) and molar mass as the same quantity Repair They share a number and nothing else. \(M_r(\mathrm{CO_2}) = 44\); \(M(\mathrm{CO_2}) = 44\ \mathrm{g\,mol^{-1}}\). Only the second one can go into \(n = m/M\). Lesson 3.2
- "\(M_r\) can go straight into \(n = m/M\)." Fix Numerically you get away with it; as a statement it is wrong, because \(M_r\) has no unit. Write the molar mass with \(\mathrm{g\,mol^{-1}}\) and the substitution becomes self-checking.
- Using the reactant's molar mass to convert the product back. Fix Stage 4 uses B's molar mass, not A's. Writing \(M(\mathrm{CaO}) = 56\ \mathrm{g\,mol^{-1}}\) on its own line, with the substance named, keeps the two apart.
- Reading \(\mathrm{g\,dm^{-3}}\) as if it were \(\mathrm{mol\,dm^{-3}}\). Fix Check the unit before choosing a route. A \(\mathrm{g\,dm^{-3}}\) value has to pass through the molar mass before it can meet an equation ratio, because ratios only speak in moles.
Questions students ask about Molar 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.