Equilibrium constant Kp

Kp, Ksp and partition

For gas equilibria, partial pressure $p_A=x_A\times P_\text{total}$, where mole fraction $x_A=n_A/n_\text{total}$. $K_p$ uses partial pressures: for $\mathrm{N_2O_4\rightleftharpoons2NO_2}$, $K_p=\frac{p_{\mathrm{NO_2}}^2}{p_{\mathrm{N_2O_4}}}$. Only temperature changes $K_p$. The solubility product $K_{sp}$ for a sparingly soluble salt, e.g. $K_{sp}=[\mathrm{Ag^+}][\mathrm{Cl^-}]$; a precipitate forms if the ionic product exceeds $K_{sp}$. Adding a common ion lowers solubility. The partition coefficient $K_{pc}=\frac{[X]_\text{solvent 1}}{[X]_\text{solvent 2}}$ describes how a solute distributes between two immiscible solvents.