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.</span>
This is because double bonds are two pairs of electrons are shared between atoms and triple bonds are three pairs, and one pair of electrons is 2, so 2 x 2=4 and 2 x 3=6.
