If l burn 6grams in plenty of air, what volume of oxygen will be used up

Answer 1

air is about 21% oxygen and 78% nitrogen, so one mole of air weighs about .21*32 + .78*28 = 28.56 grams. That means you have
0.21 moles of air.
one mole of a gas occupies 22.4L, so now you can see how much of that is O2.