Nitrogen tetroxide, N₂O₄, is a rocket propellant that can be prepared by the dimerization of nitrogen dioxide, NO₂. If you were a chemical engineer producing N₂O₄, you would need to know whether to use a high or a low pressure for the synthesis. Predict the effect of compression on the equilibrium composition of the reaction mixtures in which the equilibria 

(a) 2 NO₂(g) ⇌ N₂O₄(g) and 

(b) N₂(g) + O₂(g) ⇌ 2 NO(g) have been established.

PLAN Reaction will take place in the direction that reduces the increase in pressure, and fewer gas molecules generate a lower pressure. Therefore, compare the number of reactant gas molecules with the number of product gas molecules.