So the gaseous NO2 isn't dissolved gas in solution?
Edit: Maybe I'm stupid but I kind of assumed it was. They would have to tell us right? Or would a dissolved gas be considered aqueous? I don't think I've ever had to deal with that before.
Also BRT, I take it this means we don't have to worry about weird technicalities where a pure liquid would actually be included in the Keq expression?
This system would have to be a closed container with a gas space above the solution, so that the NO
2 could dissolve into the solution as well as be released from the solution. It seems unfamiliar in terms of chemistry, but if you think about the respiration process, it's kind of what's happening there.
In physio, we consider the PCO
2/[HCO
3-] ratio in our equilibrium expression. When CO
2 is released from the blood to the lungs, we get a complex equilibrium involving a solute and a gas.
HCO
3-
(aq) + H+
(aq) <=> H
2CO
3(aq) <=> CO
2(g) + H
2O
(g)
As we learn in physio, the pH increases from about 7.2 to about 7.4 in the process, so even though the CO
2 is a gas and not in the solution itself, it still impacts the equilibrium in the blood.
I hope this helps, because this particular passage is tough because of the large amount of reactions going on simultaneously and the unfamiliar nature of some of them.
As to when a pure liquid can be included in the equilibrium expression, at the MCAT level, it would be best to think of it as when you reach the point where the concentration is low enough that it's hard to decide whether it's a pure liquid (in contact with itself mostly) versus a solute (surrounded by other molecules). This is where the activity concept in P chem comes into play, but that's beyond the scope of the MCAT (I assume).