19. What is the correct equilibrium mass action expression for the following reaction?

Ca(HCO3)2(s) ⇔ CaO(s) + 2 CO2(g) + H2O(g)

A. [CaO][CO2][H2O]/[Ca(HCO3)2]
B. [Ca(HCO3)2]/[CaO][CO2][H2O]
C. [CaO][CO2]2[H2O]/[Ca(HCO3)2]
D. [Ca(HCO3)2]/CaO][CO2]2[H2O]
E. [CO2]2[H2O]

Mass action expression are found from the balanced chemical reaction: the product of the concentrations of products raised to the power of the stoichiometric coefficient divided by the product of the concentrations of reactants raised to the power of the stoichiometric coefficient. Pure solids and pure liquids do not appear in mass action expressions because their concentrations do not change as a function of reaction. Within these constraints, expression E is the correct response.

Comment: equilibrium constants and associated mass action expressions are an important component of understanding much of chemical reactivity.

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