The answer is B, 54 g. Reactants and products have the same mass, so if the product is 72 g of carbon dioxide, and all we know is there is 18 g of carbon, then we can find the difference of 72 and 18 by subtracting 18 from 72 which is 54 g.
From the given balanced chemical reaction, C + O2 -> CO2 the mass ratio of the carbon and oxygen that is needed for the reaction is 12:32. Simplifying the ratio gives 3 unit mass C is to 8 unit mass O2. Given 18 g of carbon, 18 g x (32 g oxygen / 12 g carbon) = 48 grams. For the given product, the ratio of oxygen to carbon dioxide is 32: 44. 72 g CO2 x (32 g O2 / 44 g CO2) = 52.36 grams
When coming to building something, some materials are more vulnerable to water and can wear away easier. Some can keep the water in, so depending on the material, the water quality can be better sustained.
