at high enough temperatures,N2(g) + O2(g)<---> 2NO (g)If this system
is at equilibrium, an increase in pressure will affect the
concentration of:
a. N2, O2, and NO.
b. O2 only.
c. NO only
d. N2 only.

Hi teatea!!

"Le Chatelier's Principle.
This is one of the most useful ways of predicting how the position of
equilibrium in a chemical reaction will change when the conditions are
changed. Le Chatelier's Principle is stated as follows:
-When the conditions of a system at equilibrium are changed, the
position of equilibrium moves in such a way as to oppose the change.

Great, but what does that mean? Le Chatelier's Principle says: 
-If we add more of any reactant or product chemical, the system reacts
in such a way to try to use it up.
-If we remove some reactant or product chemical, the system tries to
replace it by moving the equilibrium.
-If we heat up the system, it tries to cool down again. 
-If we cool it down, it tries to warm up again. 
-For reactions involving gases, if we increase the pressure, the
system then tries to reduce the pressure.
-For reactions involving gases, if we reduce the pressure, it then
tries to raise the pressure.
In every case the position of equilibrium moves to try to bring about
the appropriate change. It doesn't return to the exact same
concentration or pressure or temperature before the change was made,
it just tries to do this and partly succeeds.
Note that the equilibrium constant doesn't change, (except in the
cases where the temperature is changed of course). Instead, the
amounts of the reactants and products present change because the
equilibrium constant is a constant at any given temperature. The
equilibrium constant controls the concentrations of the reactants and
products that are present when the system comes to equilibrium.
Le Chatelier's Principle allows you to predict which way the
equilibrium will move when you change the reaction conditions, and
helps provide ways to increase the yield in a chemical reaction. In
fact every chemist has at some time in some reaction applied Le
Chatelier's Principle to improve the yield they got. That's how
important it is. Le Chatelier's Principle is a popular topic in exams,
so have a good look at these examples below, which cover the type of
things you are expected to know."
From "Le Chatelier's Principle" (see this page for examples and more
info):
http://www.sky-web.pwp.blueyonder.co.uk/Science/lechatelier.htm


If a system is at equilibrium and the pressure is increased, the
reaction will try to lessen the pressure by shifting to the side with
the lesser number of gas molecules.
In the following system: N2(g) + O2(g)<---> 2NO (g)  , neither side is
favored over the other since both sides have the same number of total
molecules (two). No matter which way the reaction shift, the total
number of molecules would remain unchanged.
In cases like this, where there is an equal number of molecules on
each side, the equilibrium would remain unchanged by the change in
pressure (in either direction).

So the correct answer is a.

I hope this helps.

Regards.
livioflores-ga