1. Avogadro's Number

* Avogadro's number tells us the number of particles (atoms, molecules, etc.) in one mole of a substance. It's approximately 6.022 x 10²³ particles/mole.

2. Setting Up the Calculation

We'll use the following relationship:

* (Number of molecules) / (Avogadro's number) = (Number of moles)

3. Calculation

* (2.045 x 10²¹ molecules O₂) / (6.022 x 10²³ molecules/mole) = 0.0034 moles O₂

Answer: There are approximately 0.0034 moles of O₂ in 2.045 x 10²¹ molecules of O₂.