For a gas PV is constant at constant temperature. If pressure goes up by ten percent, volume decreases by ten percent. Thus K is equal to one, not zero. However, for practical purposes, this value is orders of magnitude less than that for liquids and solids, so that it can be neglected, or taken to be zero.

thunderspeed,

You are right, non-zero but low values as may be seen for air in:

http://en.wikipedia.org/wiki/Bulk_modulus

Another site:

http://www-mdp.eng.cam.ac.uk/web/library/enginfo/a...

consider that bulk modulus is the inverse of isothermal compressibility,

isothermal compressibility coefficient = -(1/V)*dV/dP

or the derivative of volume vs. pressure,
bulk modulus is not 0 for real gases, see below,
for the derivative of volume vs. temperature

volumetric expansivity coefficient = (1/V) dV/dT

for mixture you can calculate these properties for both vapors and liquids
with a EOS or equivalent procedures,
see for example the Excel macros StrGIC, StrLIC, StrGVE, StrLVE in

"http://www.prode.com/en/properties.htm"

another possibility is

"http://www.nist.gov/srd/nist23.cfm"

Use "compressibility factors" for gases.

you must get smarter than the software you're using.