For gaseous water PV=nRT
Thus, volume occupied by 1 mole gaseous water can be derived as (T=273+100=373K)
V=nRTP=1×0.0821×3731=30.62litre
Also, volume of 1 mole of liquid water = mass/ density
=180.958=18−79mL=18.79×10−3litre
Thus, volume percentage occupied by water molecules in gaseous state
=18.79×10−330.62×100=0.0614
Percentage of free volume=100−0.0614=99.9386
so nearest integer value is 100.