The equation that represents the process of photosynthesis is:
6CO2+12H2O+light->C6H12O6+6O2+6H2O
Photosynthesis is the process in plants to make their food. This involves the use carbon dioxide to react with water and make sugar or glucose as the main product and oxygen as a by-product. Since we are not given the mass of CO2 in this problem, we assume that we have 1 g of CO2 available. We calculate as follows:
1 g CO2 ( 1 mol CO2 / 44.01 g CO2 ) ( 12 mol H2O / 6 mol CO2 ) ( 18.02 g / 1 mol ) = 0.82 g of H2O is needed
However, if the amount given of CO2 is not one gram, then you can simply change the starting value in the calculation and solve for the mass of water needed.