The molar mass of a chemical species is the mass of one mole of that species. For a compound, it is obtained by adding the molar-mass contributions of all atoms in its chemical formula.

The numerical atomic masses listed in the periodic table correspond, to ordinary chemical precision, to elemental molar masses in $\mathrm{g/mol}$. If a formula contains $n_i$ atoms of element $i$ with atomic molar mass $M_i$, then

$$M_{\text{compound}}=\sum_i n_iM_i.$$

For calcium nitrate,

$$\mathrm{Ca(NO_3)_2},$$

one formula unit contains one Ca, two N and six O atoms. Using

$$M_{Ca}=40.08,\quad M_N=14.01,\quad M_O=16.00\ \mathrm{g/mol},$$

we obtain

$$\begin{aligned} M_{\mathrm{Ca(NO_3)_2}} &=40.08+2(14.01)+6(16.00)\ &=164.10\ \mathrm{g/mol}. \end{aligned}$$

Therefore a $0.500,\mathrm{mol}$ sample has mass

$$m=nM=(0.500)(164.10)=82.05,\mathrm g.$$

Conversely, a measured mass can be converted to amount of substance using

$$n=\frac{m}{M}.$$

The formula determines which elemental masses must be counted; the mole converts that microscopic composition into a macroscopic mass scale.