Here's how to calculate the number of moles in 64 grams of oxygen (O2):

1. Find the molar mass of O2:

* Oxygen has an atomic mass of approximately 16 g/mol.

* O2 has two oxygen atoms, so its molar mass is 16 g/mol * 2 = 32 g/mol

2. Use the formula:

* Moles = Mass / Molar Mass

3. Calculate:

* Moles = 64 g / 32 g/mol = 2 moles

Therefore, there are 2 moles of oxygen (O2) in exactly 64 grams.