Partial Pressure Dissolved Gas

[Xo1991]

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The solubility of dissolved oxygen in water is 1.25 x 10-4 M at 25°C, where the mole fraction of oxygen is 0.21 and atmospheric pressure is 1.0 atm. In a pure oxygen atmosphere at the same pressure, what would the solubility be?

 

[Cool Beans]

This problem deals with Henry's Law, but the take away is that the concentration dissolved in the solution is directly proportional to the partial pressure of that gas in the air above it.

So in the first case you were given the mole fraction of oxygen as 0.21 and 1atm total pressure and you are supposed to deduce a partial pressure of oxygen of 0.21atm.

Then you're supposed to compare that to a pure oxygen environment where the partial pressure (technically it's all the pressure but whatever) of oxygen is 1atm.

That's roughly 5 times the partial pressure of oxygen in the second environment and so we should expect the solubility of oxygen in the solution to also increase by roughly a factor of 5:

~5(1.25x10^-4) ~ 6.25x10^-4

Essentially this is the solution to the following proportion:
x/(1atm) = (1.25x10^-4)/(0.21atm)

Hope this helps!