Molar Mass

The molar mass $M$ is the mass of one mole, in g/mol. For an element it is the relative atomic mass: carbon $12\ \mathrm{g/mol}$, oxygen $16\ \mathrm{g/mol}$.

For a compound, add the atoms of its formula: H₂O is 2 × 1 + 16 = $18\ \mathrm{g/mol}$, NaCl is 23 + 35.5 = $58.5\ \mathrm{g/mol}$.

One mole of water and one mole of salt

Same number of particles, different masses. The mass of $n$ moles is

$$m = n \times M$$