Consider the following equilibrium.

Question

2 NOCl(g) ⇌ 2 NO(g) + Cl2(g)

Determine Kc if 1.00 mol of NOCl(g) is initially placed in an empty 1.00 L flask, and 90% of the NOCl is consumed when the reaction reaches equilibrium.

Answer 1

So we know that Kc = [Cl2][NO]^2 / [NOCl]^2 based on the formula. But this is at equilibrium, so you must find the concentrations at equilibrium.
What you have to do here is calculate the intial molarity of NOCl, and then calculate the molarity of all three at equilibrium. You can use an ICE chart if that makes it easier:
2NOCl 2NO Cl2
Initial 1 0 0
Change -0.9 +0.9 +0.9
Equilibrium

Answer 2

so it will be: Kc = [Cl2][NO]^2 / [NOCl]^2
=0.9x0.9^2 / 0.9-1?

Answer 3

I don't agree with the ICE table. Also the denominator is 1-0.9 and not 0.9 -1. You can't have a negative number as a starting concentration. I would do the ICE chart like this.
............ 2NOCl(g) ⇌ 2 NO(g) + Cl2(g)
I..............1..................0...............0
C..........-0.9...............+0.9.........+0.45