[wishful]

2NOCl(g)  2NO(g) + Cl2

the temperature of the equilibrium is alterred so that the equilibrium concentrations of NOCl and NO were the same as each other.
What will be the effect on the equilibrium concentration of NOCl when :
 a mixture of NOCl(g) and NO(g) containing equal numbers of moles of each gas is introduced into the container at constant temperature?

the answer is  : no change to equilibrium position.
i dont fully understand the answer..

[Schrödinger]

What do you think will happen if
1. only NOCl is added
2. only NO is added
3. only Cl2 is added
Why? What was you basis of answering these questions?

[Juan R.]

The equilibrium constant (check a textbook if you do not know this) for your reaction is defined as:

K_eq = ( [NO]² [Cl₂] ) / [NOCl]²

Moreover you are said that [NO] = [NOCl], therefore the constant is

K_eq = [Cl₂]

What happen to K_eq if you add a "mixture of NOCl(g) and NO(g) containing equal numbers of moles of each"? The answer is nothing because in that case the equilibrium constant is independent of their concentrations.

[wishful]

yeah my internet was down for the whole weekend, so i just pick up my reference book and studied equilibria again.
I understand it now.. and thanks for the help.