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
