Number of moles of Hydrogen (H2)=Mm=216=8
Number of moles of oxygen (O2)=32128=4
Using ideal gas equation, PV=nRT,
Where pressure P=1atm=1.01×105Pa(STP)
Putting the values,
1.01×105V=(8+4)×8.314×273
V=1.01×10512×8.314×273m3
=0.27×106cm3=27×104cm3