# Using equilibrium constant in gibbs equation [duplicate]

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In my chemistry textbook, it is written that :

Gibbs energy for a reaction in which all reactants and products are in standard state, $\Delta_\mathrm{r}G^\circ$ is related to the equilibrium constant of the reaction as follows: $$\Delta_\mathrm{r}G^\circ=-RT\ln K$$

A solved example was given there for the above equation.

Calculate $\Delta_\mathrm{r}G^\circ$ for conversion of of oxygen to ozone, $$\ce{3/2O2(g)->O3(g)}$$ at $298\ \mathrm{K}$ if $K_p$ for this conversion is $2.47\times 10^{-29}$.

The answer to above problem is simple. Just substitute the values in the gibbs equation and get the value of $\Delta_\mathrm{r}G^\circ$ but the thing that I can not understand is that in the equation they substituted the value of $K_p$ directly in the equation without converting into $K_c$ for equilibrium constant.

So my question is: In the gibbs equation what does equilibrium constant stand for? ($K_c$ or $K_p$).