# In a 0.25*mol quantity of BaCO_3, how many moles of oxygen atoms are present?

If there were a 1 mol quantity of $B a C {O}_{3}$, clearly there would be 3 mol of oxygen atoms present. The question specifies a 0.25 mol quantity of $B a C {O}_{3}$, so we have 0.75 mol oxygen atoms .
If we have 0.25 mol $B a C {O}_{3}$, clearly there are 0.25 mol Ba atoms, and 0.75 mol oxygen atoms, because for every mol of Ba, there must be 3 mol oxygen atoms, and 1 mol carbon. Please note that I quote the answer in oxygen atoms , and not oxygen molecules, i.e. ${O}_{2}$ species.