The "REVERSIBLE" chemical reaction- A + B <--> C + D Kc = [C][D] / [A][B] = 6.8 Concentration at the start and after equilibrium- [A] = 2.00 M --> (2.00 - X) M [B] = 2.00 M --> (2.00 - X) M [C] = 0.00 M --> X M [D] = 0.00 M --> X M 6.8 = X^2 / (2.00 - X)^2 take the square root of both sides- 2.6077 = X / (2.00 - X) 5.2154 - 2.6077 X = X 5.2154 = 3.6077 X X = 1.4456 at equilibrium- [A] = 0.55 M [B] = 0.55 M [C] = 1.45 M [D] = 1.45 M