Final answer:
The law of conservation of mass dictates that the mass of CH4 and oxygen must equal the mass of the CO2 and H2O produced. Upon subtraction of the mass of H2O from the total mass, we find that 21 grams of CO2 should be produced, which does not match the options provided.
Step-by-step explanation:
To determine how much carbon dioxide (CO2) was created when 15 grams of CH4 (methane) reacted completely with 12 grams of oxygen, and if 6 grams of H2O (water) formed, we apply the law of conservation of mass. This law states that the mass of the reactants in a chemical reaction must equal the mass of the products. Therefore, the mass of CH4 and oxygen combined (15 grams + 12 grams = 27 grams) must be equal to the mass of the products (CO2 and H2O combined). Given that 6 grams of H2O are produced, we can subtract this from the total mass to find the mass of CO2 produced: 27 grams - 6 grams = 21 grams of CO2. However, this does not match any of the options provided (A. 3 grams, B. 6 grams, C. 9 grams, D. 12 grams), suggesting there might be an error in the question or the provided options.