How would the volume of gas produced in an experiment be affected if the pressure of the water vapor was not accounted for in the calculations?
I assume you are using PV = nRT so rearrange that to
V = nRT/PV.
Now the pressure that goes in is
room pressure - water pressure at that T. So how will the calculation of V be changed if you don't subtract th4 water pressure. Hint: you will be substituting a larger number for P instead of a smaller number that should go there.