Use the chemical equation to complete the activity.
2Cu + S -> C*u_{2}*S
Copper (Cu) reacts with sulfur (S) to form copper sulfide as shown in the equation. A scientist adds 12.7 grams of Cu to 3.2 grams of S to start the reaction. In this reaction, all of the copper and all of the sulfur react. Before running the experiment, the scientist expected the products of the reaction will be a total of 15.9 grams of copper sulfide (C*u_{2}*S) ? In 1 - 2 sentences, explain the law that the scientist used to predict that the product of the reaction would be 15.9 grams of copper sulfide.
(2 points)