Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Amount of substance
What is Amount of substance?
Amount of substance is the quantity measured in moles that counts how many specified particles are present; it is calculated from mass divided by molar mass, and it is the quantity a balanced chemical equation compares between substances.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Amount of substance 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 Amount of substance
- "A mole is a very large mass." Fix A mole is an amount of substance, not a mass. One mole of hydrogen molecules has a mass of about \(2\ \mathrm{g}\); one mole of lead atoms has a mass of about \(207\ \mathrm{g}\). Same amount, very different masses.
- "\(n = M/m\)." Fix Inverted. Substitute the units and see: \(\mathrm{g\,mol^{-1}} \div \mathrm{g} = \mathrm{mol^{-1}}\), which is not an amount of substance. The correct arrangement, \(m/M\), gives \(\mathrm{mol}\).
Questions students ask about Amount of substance
What is a mole, and why do you have to say which particle you are counting?
The mole (mol) is the unit of amount of substance. One mole contains \(6.02\times10^{23}\) particles, and that number is the Avogadro constant. The particles must be named because the count changes with the entity chosen: one mole of \(\mathrm{CO_2}\) is \(6.02\times10^{23}\) molecules but \(1.806\times10^{24}\) atoms. A mole is an amount, not a mass: one mole of hydrogen molecules has a mass of about 2 g, while one mole of lead atoms has a mass of about 207 g.