At a particular temperature, 12.0 mol of NH3 is introduced into a 2.0 container, and the NH3 partially dissociates by the reaction:
2NH3 <> N2 + 3H2
At equilibrium, 6.0 mol of H2 gas are present. Calculate the K for this reaction.
I assume that is a 2.0L container but you omitted the unit.
M NH3 = mols/L = 12.0/2.0 = 6.0M
........2NH3 ==> N2 + 3H2
I......6.0.......0.....0
C......-2x.......x.....3x
E......6.0-2x....x.....3x
Kc = (N2)(H2)^3/(NH3)^2
The problems tells you H2 at equil is 6.0mols; therefore, M H2 = 6.0/2.0 = 3.0 M. That means 3x = 3.0M or x = 1.0M
Substitute into Kc expression and solve for Kc.