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We know that

$$ΔG = ΔG^\circ + RT\ln{Q} \tag{1}$$

where $Q$ is reaction quotient, and also that at equilibrium becomes

$$ΔG^\circ = -RT\ln{k} \tag{2}$$

I was trying to apply this equation to a reaction involving gaseous products and reactants.

If $K_c = k$ for concentration and $K_p = k$ for partial pressures, and assuming the gases are ideal, we get

$$K_p = K_c (RT)^x \label{eqn:3}\tag{3}$$

where $x$ is the sum coefficients of gaseous compounds on product side minus the same on reactant side. In general, $x ≠ 0$.

Now with this background, here is my question. At equilibrium, $ΔG = 0$ and $Q = k$. I am supposed to calculate $k$ from here in a question, but is this $k$ $K_p$ or $K_c$? From $\eqref{eqn:3}$ it is obvious that only one of the $k$'s can be unity in general. So which one is it?

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