For a reaction A2+B2 ⇄ 2AB, the value of the equilibrium constant is calculated based on the concentrations of reagents and products in the equilibrium state:
Kc=[AB]2/[A2][B2].
In the case of diagramme 2, [AB]= 2[A2]=2[B2]=2c and consequently the value is: Kc=(2c)2/c.c= 4