For a reaction A(g)  + B(g)  ⇌   C(g)  +   D(g), the initial concentration of A and B are equals but the equilibrium concentration of C is twice that of equilibrium concentration of A. Then K is

A(g)  + B(g)  ⇌   C(g)  +   D(g)Initial conc.                  a           a              0              0Eq. conc.                     a - x     a - x          x              xGiven  x = 2a-x  or  x = 2a3KC = x2a-x2 = 2a/32a-2a/32 = 4