# Theoretical yields and determining the number of moles in a reaction

I'm working on a question which asks me to do two things. I've balanced the related equation as:

$$\ce{6H2O + 6CO2 -> C6H12O6 + 6O2}$$

Now, if I have $44$ grams of $\ce{CO2}$ and $36$ grams of $\ce{H2O}$, how many moles of glucose will I have? The molecular mass of $\ce{CO2}$ is $44\mathrm{~g/mole}$, and that of $\ce{H2O}$ is $18\mathrm{~g/mole}$. Do I need to determine how many moles of each element are in the certain amount of grams for each compound, and then use that to determine how many moles are in the glucose?

Secondly, for the same amounts of $\ce{CO2}$ and $\ce{H2O}$, I need to figure the theoretical yield. Am I correct in saying since for ever 6 molecules of $\ce{H2O}$ and $\ce{CO2}$ I get 1 molecule of glucose, I would do

$$36\text{ grams of } \ce{H2O} \times \left( \frac{1~\mathrm{mol}~\ce{H2O}}{18\mathrm{g}~\ce{H2O}} \right) \times \left(\frac{1~\mathrm{mol}~\ce{C6H12O6}}{6~\mathrm{mol}~\ce{H2O}}\right)$$

Then take the same for $\ce{CO2}$? I'm unsure of what to do with the values after that.

First, knowing the mass of the compounds, use the molar mass relationship for each (g/mol) to convert them all to moles. This is because when it comes to reactions, compounds react on a mole or number basis. This lets you know the "starting amounts" of $\ce{H_2O}$ and $\ce{CO_2}$. Now, our forum has a policy against providing direct answers to homework, so I'll give examples. If you hypothetically had, say, 12 moles of both water and carbon dioxide, then using the numbers in the balanced chemical equation, they can fully react to make 2 moles of glucose. Thus, 2 moles is your theoretical yield. However, in another case, if you had 9 moles of water and 14 moles of carbon dioxide, the $\ce{H_2O}$ limits the amount of glucose you can produce, and it is the limiting reactant. (14 moles of $\ce{CO_2}$ would require 14 moles of $\ce{H_2O}$. There will be some $\ce{CO_2}$ left over because you are limited by only 9 moles of $\ce{H_2O}$). In this hypothetical case, the theoretical yield would be 1.5 moles of glucose. (Your problem may or may not involve limiting/excess reactants).