A pure sample of carbon dioxide gas was obtained by strongly heating calcium carbonate.What is the ratio by mass of element present in gas?

The reaction for decomposition of CaCO3 can be written as:

CaCO3 → CaO + CO2

In CO2:

 mass of C is 12 

mass of Oxygen is = 2 x 16 = 32

Ratio of C/O = 12/32 = 3/8 or 3:8

So the element carbon and oxygen are present in ratio of 3:8 by mass.