Consider the equilibrium: $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$. The correct expression for the equilibrium constant $K_c$ is
A$K_c = [CaO][CO_2] / [CaCO_3]$
B$K_c = [CO_2]$CORRECT
C$K_c = [CO_2] / [CaCO_3]$
D$K_c = 1 / [CO_2]$
GCEChemistryChemical Equilibrium2020
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Why the answer is B, and why the others tempt you.
Pure solids have constant concentrations and are omitted from the equilibrium constant expression by convention. Since both $CaCO_3$ and $CaO$ are pure solids, only the concentration of the gaseous product $CO_2$ appears in $K_c$. Therefore $K_c = [CO_2]$.
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