At S.T.P the molar volume of a gas is 22.4L,that is 1mol=22.4l therefore the voume of 1.2 moles of water vapor is
1.2 x22.4=26.7L
alternatively
from ideal gas law v=nRT /P
At S.T.P the temperature is 273k and pressure is 1atm
R=0.0831 atm/mol/k
v=(1.2 x 0.0821 x273)/(1am)=26.8L