Answer:
9.39 x 10^23 oxygen atoms
Step-by-step explanation:
Each molecule of CaCO3 contains 3 oxygen atoms. We can determine the number of CaCO3 molecules from the 0.78 moles.
(0.78 moles)*(6.022x10^23 molecules/mole) = 4.70 x 10^23 CaCO3 molecules
(4.70 x 10^23 CaCO3 molecules)*(2 O atoms/CaCO3 molecule) = 9.39 x 10^23 oxygen atoms