Equilibrium costant

[melissa_dis4K]

In the following question I did not understand why they used both concentration and partial pressure for the equilibrium constant. I thought that you only used either concentrations or partial pressures, not both. Can someone explain to me please? This is from the sixth edition.
Self-Test 11.2A Write the equilibrium constant for the equilibrium between atmospheric
oxygen and oxygen dissolved in water, O2(g) <----> O2(aq)
[Answer: K " [O2] /PO2]
thank you!

[armintaheri]

When all your reagents are gases, you can choose whether to use partial pressures or concentrations. But in this case, you have a gas on one side and an aqueous reagent on the other. You obviously can't convert the aqueous concentration to a partial pressure since it's not a gas. And you can't convert the partial pressure to a concentration since you don't have a known volume. So the only way to do the problem is to use both partial pressure and concentration.

[melissa_dis4K]

I see.. thank you so much!