We are given the chemical reaction:
2NH3(g) = N2(g) + 3H2(g)
To determine the equilibrium concentration of hydrogen gas, we use the equation above, the Kc that is given, initiall moles of NH3 and the volume of the system. The equilibrium constant is equal to the ratio of the product of the equilibrium concentrations of the products and the equilibrium concentration of the reactant. It is expressed as:
Kc = [N2] [H2]^3 / [NH3]^2 = 4.0 x10^-2
We use the ice table:
NH3 N2 H2
I 1220 / 1 0 0
C -2x +x +3x
-----------------------------------------------------
E 1220 - 2x x 3x
Substituting to the expression of Kc,
4.0 x10^-2 = [x] [3x]^3 / [1220 - 2x]^2
Solving for x, we will have
x = 6.8336 = [N2]
3x = 3(6.8336) = 20.5008 = [H2]
Therefore, the equilibrium concentration of H2 would be 20.50 M.