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If we have the reaction: $$\ce{A +BC_{(aq)} <=> D}$$ where $\ce{A}$ and $\ce{D}$ are generic compounds (neither is pure solid nor pure liquid) and $\ce{BC}$ is an ionic compound dissociated in water (consisting of $\ce{B+}$ and $\ce{C-}$ ions), would the $K_{\mathrm{c}}$ expression be written as

$$K_{\mathrm{c}}=\frac{[\ce{D}]}{[\ce{A}][\ce{BC}]}$$

or

$$K_{\mathrm{c}}=\frac{[\ce{D}]}{[\ce{A}][\ce{B+}][\ce{C-}]}?$$

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  • $\begingroup$ The fact is that BC(aq) does not exist, so the equation is not really "correct" - we just use that as shorthand for B+(aq) + C-(aq). $\endgroup$ – orthocresol Dec 17 '15 at 22:51
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I will say the latter form is more correct, and in fact an important way to consider equilibrium with solubility and the common ion effect. You should not count any solids in equilibrium expressions though as they do not determine the final equilibrium.

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