The thermal decomposition of CaCO3 is
2CaCO3 --> 2Ca + 2C + 3O2
During a chemical reaction the total number of atoms of each element remains the same on the both the sides. This also ensures that the total mass of the reactants and the total mass of the products is the same
If 20.8 g of CaCO3 decomposes to give 10.1 g of Ca and 2.4 g of C, the missing mass in the products must be in the form of oxygen.
This gives the mass of oxygen produced as 20.8 - 2.4 - 10.1 = 8.3 g
If we'll thermal decompose the compound CaCO3, we'll get the products:
CaCO3 -> Ca + C + 3O2/2
The total mass is conserved, therefore the mass of oxygen may be determined as it follows:
Mass of O2 = Mass of CaCO3 - Mass of Ca - Mass of C
Mass of O2 = 20.8 gms of CaCO3 - 10.1gms of Ca - 2.4 gms of C
Mass of O2 = 8.3 grams of O2
The mass of the oxygen produced could be calculated also using the formula:
Mass of O2 =the number of moles * molar mass
The requested mass of oxygen produced is Mass of O2 = 8.3 grams of O2.