The following chemical equations represent three different chemical reactions. 3NaOH + H3PO4 → Na3PO4 + 3H2O 2Mg + O2 → 2MgO Pb(NO3)2 + 2NaI → PbI2 + 2NaNO3 Based on the equations, which statement is correct? Choose the correct answer. MgO is a reactant in one of the chemical reactions. MgO is a reactant in one of the chemical reactions. Na3PO4 is a product in one of the chemical reactions. Na 3 PO 4 is a product in one of the chemical reactions. A total of five oxygen atoms are involved in the first reaction. A total of five oxygen atoms are involved in the first reaction. H3PO4 shows that three atoms of oxygen are involved in the reaction.