we will assume that the only reactants are x and y and that the only product is xy.
Based on the law of mass conservation, mass is an isolated system that can neither be created nor destroyed.
Applying this concept to the chemical reaction, we will find that the total mass of the reactants must be equal to the total mass of the products,
therefore:
mass of x + mass of y = mass of xy
12.2 + mass of y = 78.9
mass of y = 78.9 - 12.2 = 66.7 grams
