Methane and oxygen react to form carbon dioxide and water. what mass of water is formed if 3.2 g of methane react with 12.8 g of oxygen to produce 8.8 g of carbon dioxide? apply the law of conservation of mass.

7.2 g of water.
       
This is a simple matter of adding up the masses of the initial reactants and determining the mass of the results. You started with 3.2 g of methane and 12.8 g of oxygen. So the total mass you start with is 3.2 + 12.8 = 16.0 grams. The reaction results in water and carbon dioxide. The mass of the carbon dioxide is 8.8 g, so the mass of the water will be 16.0 - 8.8 = 7.2 g.