[nozo]

Can someone explain to me why the equilibrium below will shift to the left if we add pressure (at constant temperature)?

Hg_(s) <----> Hg_(g)

At first, I thought there would be no change since the direction should go to where there are less moles of gas. Could it be because the equilibrium constant is simply Hg_(g) 

[Will]

Can someone explain to me why the equilibrium below will shift to the left if we add pressure (at constant temperature)?

Hg_(s) <----> Hg_(g)

At first, I thought there would be no change since the direction should go to where there are less moles of gas.

           Hg_(s) <----------> Hg_(g)
0 moles of gas <--->1 mole of gas

There are less moles of gas on the left, so the equilibrium will shift to the left. If you increase the pressure, the system will try and reduce it (Le Chatelier's Principle). Hope that helps.

[Mitch]

Which direction has less moles of gas?

[nozo]

Ohhh.... thanks