# Writing equilibrium expressions with aqueous species

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

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

• 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). – orthocresol Dec 17 '15 at 22:51