I'm going to suppose that your chemical reaction is:
[tex]2NO + O_2\ \to\ 2NO_2[/tex]
The molecular mass of NO is equal to (14 + 16) = 30 g/mol. Oxygen has a molecular mass of (16·2) = 32 g/mol. As you can see in the previous equation: 2·30 = 60 g of NO will need to 32 g of [tex]O_2[/tex] in order to complete the reaction. Let's go to do a mathematical proportion:
[tex]120\ g\ NO\cdot \frac{32\ g\ O_2}{60\ g\ NO} = \bf 64\ g\ O_2[/tex]
