Define the mole, the SI unit of chemical amount. - Leaving Cert Chemistry - Question (d) - 2011

The mole is defined as the amount of substance that contains as many elementary entities (such as atoms, molecules, or ions) as there are atoms in 0.012 kg (12 g) of carbon-12.

This quantity is equivalent to the relative formula (molecular) mass expressed in grams. Moreover, one mole contains Avogadro's number of particles, which is approximately $6.022 imes 10^{23}$ particles.