Question

why the molecular formula of zinc carbonate is ZnCO3.

Answer 1

Because in zinc carbonate, 1 zinc atom is attached with 1 carbonate ion. The formula for for zinc is Zn and carbonate ion is CO₃.
So zinc carbonate is ZnCO₃.

Answer 2

The molecular formula of any compound indicates the number of atom taking part in the formation of given compound. The number of respective atoms always written as subscript with the respective atom.

In the molecular formula of zinc carbonate, zinc always exits as $Zn^{+2} \ ion$ and carbonate ion is $\mathrm{CO}_{3}^{-2} \ ion$. Hence, to write the molecular formula, we have to cross the charges irrespective to sign.

Hence, it will be $\mathrm{Zn}_{2}\left(\mathrm{CO}_{3}\right)_{2}$. Now, writing it in the whole number ratio, we will get the formula as $\mathrm{ZnCO}_{3}$.