
Answer:
0.30
Explanation:
Step 1: Calculate the known initial and equilibrium molar concentrations
[Nâ]i = 5 mol/2 L = 2.5 M
[Clâ]i = 10 mol/2 L = 5 M
[NCl]e = 3 mol/2 L = 1.5 M
Step 2: Make an ICE chart
    Nâ(g) + Clâ(g) â 2 NCl(g)
I Â Â Â Â 2.5 Â Â Â 5 Â Â Â Â Â Â Â Â 0
C     -x     -x        +2x
E    2.5-x   5-x        2x
Step 3: Find the value of x
We know that [NCl]e = 2x = 1.5 â x = 0.75 M
Step 4: Calculate the concentrations at equilibrium
[Nâ]e = 2.5-x = 2.5-0.75 = 1.75 M
[Clâ]e = 5-x = 5-0.75 = 4.25 M
[NCl]e = 1.5 M
Step 5: Calculate the equilibrium constant (Kc)
Kc = [NCl]² / [Nâ] Ă [Clâ]
Kc = 1.5² / 1.75 à 4.25 = 0.30