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.

as given the equation is

2al + 6 hcl > 2 alcl3 + 3 h2(g)

as per equation two moles of aluminum reacts with six moles of hcl to give two moles of alcl3 and three moles of hydrogen gas

given: three moles of al are present and 7 moles of hcl

for three moles of aluminum nine moles of hcl will be required

however as 7 moles of hcl are present so the limiting reagent is hcl

hcl is going to decide the number of moles of product formed

when six moles of hcl are present = 3 moles of h2 are formed

when one mole of hcl is present == 3/6 mole sof h2 will be formed

when 7 moles of hcl are present = 3 x 7/ 6 moles of h2 will be formed = 3.5 moles of h2