Which quantity of o2 contains exactly 3.01 x 10^23 molecules?

0.5 moles or 16 grams

Explanation:

The Avogadro's number links the number of elementary particles (like molecules) with a mole of substance (here O2). So:

number of moles = number of molecules/Avogadro's number

number of moles =  3.01 x 10^23 molecules/(6.023 x 10^23 molecules/mole) = 0.5 moles

To convert moles into mass we need the molar mass of the substance.

Molar mass of O2 = 32 grams/mole

Now we can compute the mass as follows:

mass = number of moles * molar mass

mass = 0.5 moles * 32 grams/mole = 16 grams

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