The mole and the Avogadro constant — FAQ

Carbon-12 was chosen as the standard because it is a common, stable, and easily purified solid. Using a specific isotope (carbon-12) provides a precise and reproducible standard for defining the atomic mass unit and, by extension, the mole.

Relative molecular mass ($M_r$) is a ratio and has no units. It's the mass of a molecule compared to 1/12th the mass of a carbon-12 atom. Molar mass ($M$) is the mass of one mole of a substance and has units of g mol^{-1}. Numerically, they are the same value.

Yes, if you want the final concentration to be in the standard units of mol dm^{-3}. The formula $c = n/V$ requires volume ($V$) to be in $\mathrm{dm}^{3} to give concentration ($c$) in mol dm^{-3}$. It's a critical step that is often tested.

No, absolutely not. The molar volume of 24 $\mathrm{dm}^{3} mol^{-1} applies only to gases at room temperature and pressure$. The volume of one mole of a liquid or solid is much smaller and depends on its density. For example, one mole of water (18 g) has a volume of only about 18 $\mathrm{cm}^{3}$.