Here's why:

* TiO2 is a compound, not a mixture. This means the titanium (Ti) and oxygen (O) atoms are chemically bonded together in a fixed ratio.

* The chemical formula TiO2 indicates that each molecule contains one titanium atom and two oxygen atoms.

* Oxygen molecules (O2) consist of two oxygen atoms bonded together. These molecules are found in the air we breathe.

Therefore, TiO2 contains two oxygen atoms, but they are not present as O2 molecules.