Subject: Why don't we make a volume correction when a gas is collected over water?

If a gas is collected over water, all textbooks show the pressure correction by 
subtracting the vapor pressure of water.  But why no volume correction?  The 
water vapor that is present is responsible for some of the volume that is 
occupied.  The gas that is of interest would occupy a smaller volume if not for 
the water vapor.  This would give a smaller answer when we use n = PV/RT.  This 
is complicated because we can't get the # of mols of water vapor present from 
the info usually given in the problems.  

Re: Why don't we make a volume correction when a gas is collected over water?