According to the law of conservation of mass, the total mass of the reactants must equal the total mass of the products in a chemical reaction.
In this reaction, 2Hg + O2 → 2HgO, we know that the total mass of the products (HgO) is 433.2 grams.
Therefore, the total mass of the reactants (mercury and oxygen) is also 433.2 grams.
Answer: 433.2 grams