On Sun, 23 Feb 1997, Tom Gehring wrote:
> Partial credit to a couple of the previous writers. But water neither
> displaces air nor increases oxygen content. For a given temperature and
> atmospheric pressure a cubic foot of air contains the same amount of
> oxygen no matter how much water it has in it. However, as an earlier
> Tom Gehring
> Chemical Engineer & '65 MGB owner
Tom,
I hesitate to dispute a Chemical Engineer, but what about Dalton's Law?
Each gas in a mixture exerts a pressure according to its own
concentration, independently of the other gases present. That is, each
component of the mixture behaves as though it were present alone. The
pressure of each gas is referred to as its partial pressure. The total
pressure of the gas is the sum of the partial pressures of all gases
present. The partial pressure of oxygen in the atmosphere is:
PO2 = P x FO2,
where P is atmospheric pressure and FO2 is the fraction of the mixture
that is oxygen, usually around 0.21. Conventionally, the fractional
concentration refers to dry gas. In gas that contains water vapor, the
water vapor pressure has to be subtracted from P. If, for example, the
water vapor pressure were 20 mm mercury, then the equation would be:
PO2 = (P - 20) x FO2
For a fixed pressure P, the partial pressure of oxygen would be less in
humid air than in dry air. I contend, then, that a cubic foot of dry air
contains more O2 than a cubic foot of humid air, if the pressures are the
same.
Ray
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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