Final answer:
The standard entropy change for the given reaction cannot be calculated due to missing standard entropy data.
Step-by-step explanation:
The standard entropy change (ΔS°) for a reaction can be calculated by summing the standard entropies of the products and subtracting the sum of the standard entropies of the reactants. In this case, the reaction is:
MgCl₂ (s) + H₂O (l) → MgO(s) + 2HCl(g)
To calculate the ΔS°, we need the standard entropies of each substance involved. Unfortunately, the provided information does not include the standard entropy data for MgCl₂, MgO, or HCl. Without this information, we cannot calculate the standard entropy change for the reaction at 25°C.