Question

State why the molecular formula of zinc carbonate is'ZnCO3'.

Answer 1

the valency of

1) Zn is 2
2) CO3 (radical) is 2

we know that in a molecular formula the Valencies of the compounds gets interchanged.

here both have same valency

so the valencies get cancel and the formula becomes ZnCO3

Answer 2

zinc  ---- Zn 2+

carbonate ---- (CO3) 2-

in the molecular formulas the valency will get interchanged as both have the number 2 , the +ve and - ve will cancel out 

hence the molecular formula of zinc carbonate is 'ZnCO3'