Answer: the volume of the unknown mass
Explanation:
The other choices, molar masses of the reactants, the balanced chemical equation, and the molar masses of the product, are essential for the realization of stoichiometric calculations.
The balanced chemical equations tells the mole ratio of reactants (calcium carbonate) and products (calcium oxide and carbon dioxide).
The molar masses of reactants are used to convert between mass (grams) and moles.
The volume of the unknown mass is not needed when you are working only masses and moles since the relation is given by the molar masses. The volume is required when you are working with gas pressures, molarity contentrations or the information has to be worked from density measures.
