How can i determine that the volume is decreasing?

Question

Of the following equilibria, only ??? will shift to the left in response to a decrease in volume.

A) 2HI <->(g) H2 (g) + I2 (g)
B) N2 (g) + 3 H2 (g)<-> 2 NH3 (g)
C) 4 Fe (s) + 3 O2 (g)<-> 2 Fe2O3 (s)
D) 2 SO3(g)<-> 2 SO2 (g) + O2 (g)
E) H2 (g) + Cl2 (g) <-> 2 HCl (g)

Answer 1

The easy way to do is to say, "volume gos down, pressure goes ??. Of course the answer is up. All of these are gaseous reactions, so an increase in P will cause the equilibrium to shift to the side with the smaller number of moles of gas. So you look at each one.
A no change in number of moles so unaffected.
B. P goes up, rxn shift to the right.
C. P goes up, rxn shifts to the right.
D. P goes up, rxn shifts to the left.
E. P goes up, no change.