Question 3
What is the mass of one mole of oxygen molecules (O2)? (Ar: O = 16)
The right answer is the one marked above.
32 g. The relative molecular mass of O2 is 2 × 16 = 32, so one mole of molecules is 32 g.
16 g is the mass of one mole of oxygen atoms. The question asks about molecules, so the subscript 2 counts — always check "a mole of what".