What is the mass of two moles of oxygen gas $$(O_2)$$?
One mole of oxygen is $$15.9994\ g$$. but because oxygen is a diatomic molecule, it is bonded with itself.
So, $$15.9994\ g\times 2=31.9988\ g$$ per mole of oxygen.
There are two moles oxygen. So, the answer is $$63.9976\ g$$.