The formula unit mass of calcium carbonate (${\mathrm{CaCO}}_3$) is:

To calculate the formula unit mass, we add up the atomic masses of constituent elements of the given molecule.

$$\begin{gathered} \mathrm{Molecular} \mathrm{mass} \mathrm{of} {\mathrm{CaCO}}_3= ( \mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{Ca} +\mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{C}+ 3\times \mathrm{Atomic} \mathrm{mass} \mathrm{of} \mathrm{oxygen}) \\ = (40 + 12 + 3\times 16 ) \\ = ( 52 \mathrm{g}/\mathrm{mol}+ 48 \mathrm{g}/\mathrm{mol} ) \\ = 100 \mathrm{g}/\mathrm{mol} \end{gathered}$$