Final answer:
To find the standard entropy change (ΔS°) for the formation of one mole of gas from its elements, subtract the standard entropies of reactants from the standard entropies of products, after multiplying them by their stoichiometric coefficients.
Step-by-step explanation:
To calculate the standard entropy change (ΔS°) of the formation of one mole of gas from its elements, we adhere to the rule of 'products minus reactants'. Specifically, ΔS° is found by taking the sum of the standard entropies of the products, each multiplied by their stoichiometric coefficient in the balanced equation, and subtracting from it the sum of the standard entropies of the reactants, also multiplied by their respective stoichiometric coefficients. This calculation is represented by the equation ΔS° = ΣvS°(products) − ΣvS°(reactants), where 'v' is the stoichiometric coefficient.
Therefore, the correct answer to the student's question is A) Subtract the standard entropy of reactants from products.