For the reaction,
#A + B rightleftharpoons C+ D#, we can write the equilibrium constant in terms of concentration, #mol*L^-1#:
#K_c# #=# #([C][D])/([A][B])#
Note that #K_c# is dimensionless. Should the reaction be in the gas phase, then we can utilize the Ideal Gas Law: #PV=nRT#, or #n/v = P/(RT)#. Dalton's Law of partial pressures holds that the pressure exerted by a component in a gaseous mixtures is the same as the pressure it would exert if it alone occupied the container, thus,
#n_A/v = P_A/(RT)#; #n_B/v = P_B/(RT)# etc.
And #K_P# #=# #([P_C][P_D])/([P_A][P_b])# #=# #K_c/(RT)#
Reasonably, ideal gas behavious is assumed. So, the units can be in #mol*L^-1#.
