The reaction below is exothermic: 2so2 (g) o2 (g) 2so3 (g) le châtelier's principle predicts that will result in an increase in the number of moles of so3 (g) in the reaction container.

The balanced chemical reaction is as given above,

                         2SO2(g) + O2(g) --> 2SO3(g)

This is a reversible reaction. According to Le Chatelier's principle, the things that would allow in the increase of the number of moles of SO3(g) is,

(1) increase in the amount of the reactants which are SO2 and O2
(2) decrease in volume of the vessel to favor only 2 moles SO3 compared to 3 moles when combining 2 moles of SO2 and 1 mole of O2. 
(3) The effect of raising the temperature can be determined if we are given if the reaction is exothermic or endothermic. 

The correct answer is "atomic weight".