6CO2  + 6H2O  —>  C6H12O6  +  6O2   
If 12 moles of oxygen were produced, how many moles of carbon dioxide were used? 

12 number of moles of carbon dioxide are used.

Explanation:

Given data:

Number of moles of oxygen produced = 12 mol

Number of moles of CO₂ used = ?

Solution:

Chemical equation:

6CO₂ + 6H₂O      →     C₆H₁₂O₆ + 6O₂

Now we will compare the moles of oxygen with carbon dioxide.

                     O₂            :          CO₂

                      6             :             6

                    12              :             12

Thus, 12 number of moles of carbon dioxide are used.