[cmquer]

At equilibrium state, when there is the disturbance of concentration of the reactant, decrease [ reactant ], there's net direction toward formation of the reactant.

I want to know if the concentration of the product decreases or not at the same time. THANK YOU!

[Borek]

This will be described by the mass conservation and reaction stoichiometry.

[cmquer]

This will be described by the mass conservation and reaction stoichiometry.

So, the concentration of the product decreases too, right?

More,  2H2S (g) + O2 (g) <-- --> 2S (s) + 2H2O (g)
if O2 is added, [H2O] increases, how can I explain this?

[Kryolith]

Try to look at

http://en.wikipedia.org/wiki/Le_Chatelier%27s_principle

There you'll find the influence of the concentration and an explanation