A substance which has face-centred cubic crystal has a density of $1.984\mathrm{g}{\mathrm{cm}}^{-3}$ and the edge length of unit cell equal to 630 pm. The molar mass of the substance is

$$\begin{gathered} M=\frac{\rho a^3{N}_{\mathrm{A}}}{N} \\ =\frac{\left(1.984\mathrm{g}{\mathrm{cm}}^{-3}\right)\left(630\times {10}^{-10}{\mathrm{cm}}^3\right)\left(6.022\times {10}^{23}{\mathrm{mol}}^{-1}\right)}{4} \\ =74.7\mathrm{g}{\mathrm{mol}}^{-1} \end{gathered}$$

The total mass in grammes of all the atoms needed to form a molecule per mole is what makes up the molar mass, also known as the molecular weight. Grams per mole is the unit of molar mass.