A gas containing air and water vapor is in equilibrium with a liquid at 39 C and 1 bar pressure . While the liquid is nearly pure water, an engineer wishes to determine the trace amounts of oxygen and nitrogen dissolved in the liquid and also the amount of water vapor in the gas phase.

The mole fraction of water in the gas phase at this temperature is 0.0699

Next we need the constants for Henry's Law.
It may be useful to consult Perry’s handbook Perry's Handbook at UIUC or the NIST web site NIST Webbook

The NIST site uses a slightly different notation, but we can easily convert their data to compute the value of Henry's Law constant consistent with our lectures.

NIST gives data for the prediction of a coefficient over a range of temperatures , where and the units of are (mol/kg bar). In effect, it is used in Henry's law to calculate a measure of A in units of mols/kg solvent.

In our version of Henry's Law, we want mole fraction = (mols of A)/(mols of mixture). We need to use MW to convert from NIST's data to mole fraction.. Since the solute has only a VERY small mole fraction, the average MW of the liquid mxture is essentially that of pure water, and we use MW of water (18 g/mol or .018 kg/mol). Thus, with NIST's we calculate the Henry's law coefficient in units of where .

Bottomline, the equation we need from NIST is


As an example, for oxygen, NIST gives
= .0013 and C=1700

What are the Henry’s Law constants (in ) for nitrogen and oxygen in water at 39 ℃ ?


What are the mole fractions of oxgen and of nitrogen in the liquid phase at this temperature?

NOTE: small numbers for may be entered as .00001234 or 1.234e-5