Final answer:
The correct equilibrium constant expression for the reaction MgCl₂·H₂O → MgO + 2HCl is K = [HCl]^2, as MgCl₂·H₂O and MgO are solids and not included in the expression.
Step-by-step explanation:
The equilibrium constant expression for the reaction MgCl₂·H₂O → MgO + 2HCl would be based on the concentrations of the products raised to the power of their stoichiometric coefficients, divided by the concentrations of the reactants also raised to the power of their stoichiometric coefficients.
Since MgCl₂·H₂O is a solid and MgO is also likely to be a solid under standard conditions, we would exclude them from the equilibrium expression because the activity of pure solids is taken as 1. Therefore, the correct expression would only include the aqueous or gaseous components, in this case, HCl(aq).
The correct equilibrium expression would be K = [HCl]^2. This is because in the reaction, MgCl₂·H₂O is a solid and its concentration is not included in the equilibrium expression, similarly MgO is also a solid and therefore would not appear in the equilibrium constant expression.