Question

How does the mole relate to carbon-12?

Answer 1

Mole is simply a unit for counting entities at the microscopic level (that is atoms, molecules, particles, ions etc.) just as we use one dozen for twelve objects, in SI system, mole was introduced as seventh base quantity for an amount of a substance.

Complete step-by-step answer: One mole is defined “the amount of a substance that contains as many particles or entities as there are atoms in exactly $12$g (or $0.0012$kg) of the $12$ C isotope of carbon”. The mole of a substance always contains the same number of entities, no matter what the substance may be.
In order, to determine this number accurately the mass of a Carbon-$12$ atom was determined by a mass spectrometer and found to be equal to $1.992648\times {{10}^{-23}}g$
Knowing that one mole of carbon weighs $12$ grams the number of atoms in it can be calculated as follows:
$\dfrac{12\,g/mol\,{{C}^{12}}}{1.992648 \times {{10}^{-23}}g/{{C}^{12}}atom}=\,6.0221367 \times {{10}^{23}}$ atoms/mole.
The number $6.022\times {{10}^{23}}$ is given a separate name and symbol, known as Avogadro constant, denoted by $\left( {{N}_{A}} \right)$ in honor of the great scientist ‘’Amedeo Avogadro’’.
Therefore, we can say that
$1\,mole=\,6.022\times {{10}^{23}}$ particles.
For example,
One mole of oxygen atoms= $\,6.022\times {{10}^{23}}$ atoms
One mole of oxygen molecules=$\,6.022\times {{10}^{23}}$ molecule
One mole of aluminum molecules=$\,6.022\times {{10}^{23}}$ molecules
One mole of cyanide ions= $\,6.022\times {{10}^{23}}$ ions
Hence, we can say that isotope of carbon C-12 was used to determine the quantity which is represented by one mole of any entity.

Note: The average relative mass of a molecule of the substance as compared with mass of a carbon atom of carbon-12 taken as 12u is termed as the molecular mass of the compound. It should also be noted that one mole of any substance at STP contains volume equal to $22.4$ L.