if a reaction as such : X(s) + Y(g) --> Z(g)

 

then K(eq)= [z]/[x][y]

conc of x will be the no. density/molar mass , which is a constant  thus conc of x will not vary so we merge Keq with other const to  Kc=[Z]/[Y]

 

 

hope i am rite plz tell if i am wrong