The thermal decomposition of 20.8 g of CaCO3 produces 10.1 g of Ca and 2.4 g of C. What is the mass of oxygen produced?

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.