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

Joseph,

     First, let me say that your question is a good one that shows good 
insight.  I shall give an answer in two parts.  The first part is the 
simple answer:  one does not have to make a volume correction for water 
vapor volume when collecting a gas over water because the water vapor 
pressure incorporates the water vapor volume.

     The more involved answer goes something like this:  The volume of 
water vapor that is part of the gas mixture for a gas collected by water 
displacement is accounted for in correcting for the pressure exerted by 
the water vapor according to Dalton's law of partial pressures.  Where PT 
(P sub T...sorry, for some reason my "cut and paste" command does not 
work, therefore the awkward appearance) is the total pressure of a mixture 
of gases and P1, P2, P3, ... Pn are the partial pressures of the gases in 
the mixture, Dalton's law of partial pressures states mathematically that
Pt = P1 + P2 + P3 + ... + Pn.

     Considering each gas in the mixture and applyiing the ideal gas 
equation, PV = nRT, where P is the pressure, V is the volume occupied by a 
gas, n is the number of moles, R is the ideal gas constant, and T is the 
Kelvin temperature, Dalton's law may be re-written as:

            PT = n1RT1/V1 + n2RT2/v2 + n3RT3/V3 + ... + nnRTn/Vn

So, it can be seen from the above that the partial pressure of each gas in 
the mixture has the volume inherently incorporated therein.  Therefore, 
when correcting for the partial pressure exerted by water vapor in a gas 
collected over water, there is no need to account for the water vapor 
volume.

     Now, I will be quick to add that this is the argument that I put 
forth as an explanation to your query and, to paraphrase the late, great 
physicist Niels Bohr, "You should not take anything I say as an assertion, 
but as a question designed to seek the truth."  And, "That which is simple 
really has not been studied thoroughly."---Unknown.

References:  (1) H.D. Crockford and Samuel B. Knight, FUNDAMENTALS OF 
                 PHYSICAL CHEMISTRY, 2nd Ed., pp. 16-7, John Wiley and 
                 Sons, Inc., New York, 1966.
             (2) Virginia R. Williams, et al., BASIC PHYSICAL CHEMISTRY
                 FOR THE LIFE SCIENCES, 3rd Ed., p. 30, W. H. Freeman
                 and Co., San Francisco, 1978.
             (3) Cedric Stratton, Ph. D., Professor Emeritus of Physical
                 Chemistry, Armstrong Atlantic State University, Savannah,
                 GA, conversation regarding Dalton's law of partial 
                 pressures, March 15, 2001.

Sincerely,
Charles Riner