Chemistry - Equilibrium

[moxy]

1. The problem statement, all variables and given/known data

Which of the following equilibria is least affected by a change in the volume of the system?

A. 2 C (s) + O2 (g) = 2 CO (g)
B. 2 NO2 (g) = N2O4 (g)
C. H2 (g) + S (l) = H2S (g)
D. H2O (l) = H2O (g)
E. 2 NO (g) + Cl2 (g) = 2 NOCl (g)


2. Relevant equations

N/A

3. The attempt at a solution

I know the answer is C, but I don't know why. I thought maybe it had something to do with the number of moles on each side of the reaction. As volume decreases, pressure increases, and the reactions will favor the side with the least number of moles. I know this is true, but perhaps it doesn't apply? This is a sample exam problem, and any hints as to how I should approach it would be greatly appreciated.

[kuahji]

This deals with the Le Chatelier principle.

A. 2 C (s) + O2 (g) = 2 CO (g)
B. 2 NO2 (g) = N2O4 (g)
C. H2 (g) + S (l) = H2S (g)
D. H2O (l) = H2O (g)
E. 2 NO (g) + Cl2 (g) = 2 NOCl (g)

Take A for example. If you increase the decrease the volume, the reaction will shift left, because there are less moles of gas on that side. The change in volume usually only affects gases. So if you were to have an equal number moles of gases on both sides of the reaction, changing the volume wouldn't really affect the system. I think the only key thing you're missing is moles of "gases."

[moxy]

Ah, you save the day again. The gas part went over my head. This makes perfect sense now. Thanks...again!