[harini_5]

In the chapter of chemical equilibrium I’m learning a relation between ∆Go and equilibrium constant K which is, ∆Go = -RTlnK
But when to substitute K as Kp, Kc or Kx?

[Yggdrasil]

This equation gives the value of K_c

[harini_5]

This equation gives the value of K_c

But a few books like O.P.Agarwal,"IIT CHEMISTRY" says it is K_p . my text book says the usage depends upon the CONTEXT but I'm not able to get any conceptual clarity.

[Yggdrasil]

Actually, I was mistaken.  For reactions involving ideal gasses, ∆Go = -RTlnK_p.   ∆Ao = -RTlnK_c, where A is the helmholtz free energy.  The explanation why requires some advanced knowledge of statistical thermodynamics, but it has do do with the special properties of the Gibbs free energy under conditions of fixed temperature and pressure, and the special properties of the Helmholtz free energy under conditions of fixed temperature and volume.

[harini_5]

So what to conclude?Is there any hint that I could get from my question that would suggest which equilibrium constant to be used?

[Yggdrasil]

It depends on what kind of reaction you are looking at.  For a reaction involving ideal gasses, ∆Go = -RTlnKp.  For a reaction involving solutes in an ideal solution, ∆Go = -RTlnKc.