Cambridge O Level Chemistry · Syllabus 5070 · Stoichiometry
Avogadro constant
What is Avogadro constant?
The Avogadro constant is the number of specified particles in one mole of a substance, taken as 6.02 times 10 to the 23 per mole in Cambridge O Level Chemistry; it converts between amount of substance in moles and number of particles.
This definition is part of the Stoichiometry chapter in Cambridge O Level Chemistry.
Avogadro constant 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.
Questions students ask about Avogadro constant
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.