The mole concept

The mole makes it possible to equate the number of particles with a mass that can be measured.

The mole is the unit of amount

• The amount of substance $(n)$ is measured in moles $\rm (mol)$.
• The mole concept applies to all species: atoms, molecules, ions, electrons, formula units.
• $\rm 1~mol$ contains the same number of chemical species as there are atoms in exactly $\rm 12~g$ of the isotope carbon$-12$, $\rm ^{12}_6 C$.
• $\rm 1~mol$ of any substance contains $6.02 \times 10^{23}$ species.
• $\rm 6.02 \times 10^{23}~mol^{–1}$ is called Avogadro’s constant $\rm (L)$. It has units $\rm (mol^{–1})$ as it is the number of particles per mole.
• The relative atomic mass $(\mathrm A_r)$ of an element is the average mass of an atom according to relative abundances of its isotopes, on a scale where the mass of one atom of $\rm ^{12}_6 C$ is $12$ exactly. It has no units.

Empirical and molecular formula

• The relative molecular mass $(\mathrm M_r)$ is the sum of the relative atomic masses of the atoms in the molecular formula.
• The relative formula mass of an ionic compound is the sum of the relative atomic masses of the ions in the formula.
• The molar mass $\rm (M)$ is the relative mass expressed in $\rm g$ and has units of $\rm g~mol^{–1}$.
• The empirical formula gives the ratio of the atoms of different elements in a compound. It is the molecular formula expressed as its simplest ratio.
• The molecular formula is a whole-number multiple of the empirical formula.
• The empirical formula of a compound containing the elements $\rm X$, $\rm Y$ and $\rm Z$ can be determined by completing the following table.

• The molecular formula shows the number of atoms of each element present in a molecule.
• Number of mol $=$ mass/molar mass: $\rm n = m/M$
• Number of particles $=$ number of mol $\times$ Avogadro’s constant: $\rm N = nL$