If 4.0 mol of NO and 4.0 mol of O2 are combined, how many moles of NO2 can be produced?
2NO + O2 —> 2NO2

There will be 4 moles of NO2 produces at the end of the reaction.

Explanation:

First of all, you should consider that the limiting reactant is NO, since it's the first it's going to end in the reaction.

After that, the conversion factor must be done with the limiting reactant (NO). So since you have 4 mol of NO and the relation of NO with NO2 its 2:2, you will end up having 4 moles of NO2.