I think the correct term to fill in the blank would be equal. According to the law of conservation of mass, when sodium, hydrogen, and oxygen react to form a compound, the mass of the compound is equal the sum of the masses of the individual elements. This particular states that the mass cannot be created nor destroyed. So, for every process, the total inflow of mass should be the same total mass for the outflow since mass should be constant all through out. So, for a reaction, the total mass of the reactants should be equal to the total mass of the products.
