Stoichiometric calculations are not possible when:

Explanation

The correct answer is D. Side reactions occur alongside the main reaction and can alter the expected outcomes by producing additional products or consuming reactants not anticipated by the primary balanced chemical equation. This disrupts the stoichiometric calculations as the actual chemical species' amounts differ from those predicted.Option A is incorrect because when all reactants are converted into products completely, stoichiometric calculations are straightforward and precise.Option B is incorrect as the absence of reversible reactions allows the reaction to proceed in one direction, simplifying stoichiometry.Option C is incorrect because adherence to the laws of conservation of mass and definite proportions ensures accurate stoichiometric calculations.