2Mg + O2 --> 2MgO

Question

REMEMBER our mole ratio is 2Mg:1O2, 2Mg:2MgO, 1O2:2MgO

So if we had 3Mg how much O2?
(3Mg)(1O2/2Mg) = 1.5O2

How much MgO could be produced from 3O2?

Answer 1

you have two Mg for every one O2
so
mols Mg/mols O2 = 2/1

3 mols Mg ( 1 mol O2 / 2 mols Mg) = 3/2 = 1.5 mols O2 ----- Yes

1 O2/2 MgO

3 O2 ( 2 MgO/1 O2 ) = 6 MgO