Answer: The volume of water vapor produced is 26.6 L.
Step-by-step explanation:
To calculate the volume of water vapor when number of moles changes , we use Avogadro's Law. This law states that the volume is directly proportional to the number of moles of gas if temperature and pressure remains constant.
Mathematically,

where, 
 = Initial volume and number of moles
 = Initial volume and number of moles
 = Final volume and number of moles
 = Final volume and number of moles
For the given chemical reaction:

By Stoichiometry,
1 mole of methane reacts with 2 moles of oxygen gas to produce 1 mole oc carbon dioxide and 2 moles of water vapor.
We are given: 

Putting values in above equation, we get:


Hence, the volume of water vapor produced is 26.6 L.