Question

2Cu+S→Cu2S

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 (Cu2S). 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
Answer in 7th graders words in 3 small sentences

Answer 1

The scientist used the law of conservation of mass. It says matter is not created or destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products. Since 12.7 g Cu + 3.2 g S = 15.9 g, the copper sulfide must weigh 15.9 g.