The subscripts in the formula for a substance indicate both
- the relative numbers of the atoms in one formula unit of the substance, and
- the relative numbers of moles of those atoms in one mole of the substance.
The molar mass (M) is the mass of one mole of a substance and has the unit g mol–1.
The molar mass for a substance can be calculated from its formula and the molar masses of the atoms comprising the substance.
The relative atomic masses appearing under the symbol for the element in the Periodic Table equal the molar mass of atoms of that element.
Thus
M(MgCl
2) = (24.3 + 2 × 35.5) g mol
–1 = 95.3 g mol
–1