Oxygen makes up ##48%## of the mass of ##CaCO_3##

To calculate by mass, you start by considering one mole of the given compound. The mass of one mole of calcium carbonate is 100.1 g

Of that mass,

40.1 g is due to the one mole of calcium atoms it contains.

12.0 g is due to the one mole of carbon in it, and

48.0 g is due to the three moles of oxygen atoms in one mole of ##CaCO_3##

Now. to find percent composition, you take each element separately, and do this calculation:

##(“mass of element” )/(“molar mass of compound”) xx 100##

So, for oxygen, this would be

##(48.0)/(100.1) xx100=48.0%##